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close this bookAnimal Traction in Rainfed Agriculture in Africa and South America (GTZ, 1991, 311 p.)
close this folderE. Aspects of implement use
View the document1. Labour productivity and distribution
View the document2. General features of the implements
View the document3. Field preparation
View the document4. Soil preparation
View the document5. Implements for soil preparation
View the document6. Seeding
View the document7. Weed control

1. Labour productivity and distribution

1.1 Productivity

Animal traction serves especially the purpose of reducing work peaks by means of mechanizable work operations, expanding the cropping area, making the work easier or facilitating the cultivation of heavy soils (e.g. river valley bottoms). According to Pingali et al. (1987) mechanization initially occurs with energy-intensive and -for the farmer -toilsome work operations.

Thus, the draft animals are generally first employed for seedbed preparation (transition from hand hoe to the plow) and for transportation. In the first years following their introduction the draft animals are only used for a few work operations, so that the labour productivity (yield per labourer) does not increase, in comparison to the farms without animal traction. The labour-saving effect is at first often cancelled out by more thorough land clearing (destumping) and animal husbandry.

Also the area productivity (yield per hectare) generally does not increase alone by means of mechanization. In comparison to soil tillage with the hoe the yield due to plowing does not expand (Pingali et al., 1987; Strubenhoff, 1988; see section C 4.1). The area productivity can actually decline due to a reduction of mixed cropping. Planting can follow a certain time plan more accurately, however this advantage is temporarily eliminated during the period of expansion of the area under crops. On the other hand, more rapid and frequent weeding with draft animals can increase the yield considerably, although one can generally accomplish more thorough work with the hand hoe.

Increasing intensity of draft-animal use, also for all work operations, considerably increases the benefits of draft-animal technology. In drier regions the increase of area performance for cultivation and seeding at the start of the rainy season to reduce the risk is particularly important. Pre-conditions are however that

- the cropping area is not expanded,

- the danger of animal loss is low,

- the implements are reliable and do not represent an additional risk by down-time due to poor spare part and material supplies.

If the vegetation period is too short the implements cannot be fully exploited and their purchase is not worthwhile. Animal traction can then only be introduced under certain conditions, such as for light soils and low potential of weed invasion (e.g. in Senegal), via direct seeding with an animal-drawn seeder (see section F 3, case study: Senegal).

In part, profitability is first achieved with additional hiring out of the animals and the implements (Kirk, 1987; Strubenhoff, 1988). This can include assistance to relatives or neighbours as well as wage labour or labour exchange, for example plowing against hoeing weeds. This possibility only exists as long as a larger proportion of farmers do not own draft animals. The danger exists that the social position of a few draft-animal farmers will be improved at the cost of the hand-hoe farms, as has been reported from Zambia (Kurbjuweit, 1989). In the survey it was found that hiring out occurs in approximately half the cases. The farm areas in the draft-animal regions predominantly have a size of between 1 and 10 ha (86 %), close to three-quarters are smaller than 5 ha The plot sizes usually are between 0.2 and 1.0 ha.

In 73 % of the cases the draft-animal farms are being expanded. If animal-traction measures are in the introductory phase of a development project and are still limited to the farmers under the supervision of the project, then an expansion is occurring in all the cases according to the survey. In regions having permanent cropping and better developed mechanization the expansion is found to a lesser extent. These areas are characterized more frequently by high land-use intensity, distribution and tradition of animal traction, greater mechanization of work operations such as weeding and seeding as well as the employment of tractors.

Particularly in the initial phase of draft-animal use it must be reckoned that cropping areas will expand, whereby either new areas are cropped, the fallow is reduced or a redistribution of land to the advantage of draft-animal farms takes place. The maximum area that can be cultivated by draft animals depends, among other things, on the duration of the vegetation period and the daily working time of the team. In North Cameroon the limit is 3.2 ha per season for plowing and 4.4 ha for ridging (Kirk, 1987). Here the teams are used daily for 5 hours. Strubenhoff (1988) assumes a maximum of draft-animal labour to be 400 hours in Togo. On the basis of pure calculations this limit would be achieved with the cropping area of 10 ha (plow) and 15 ha (ridger). Due to the time limitation for seedbed preparation because of the short growth period the capacity limit should already have been achieved with 8 ha. In humid areas, for example in South Brazil, a greater cropping area can be cultivated.

There a transition to the tractor takes place above 15 ha cropping area. It is interesting that the profitability of motor mechanization in rainfed cropping in this region is only achieved with 35 ha of mechanizable cropping area (double-axle tractor, no vegetable cropping) (Klingensteiner, 1987). Between the maximum area of 10 -15 ha, which can be cultivated with a span, and the mentioned 35 ha there exists a large gap that, considering low wages and world market prices for agricultural products, can only be closed by hiring out, if state subsidies are not included. The economic minimum is set at about 5 ha cropping area in Togo, which also applies for North Cameroon (climate: semihumid/semiarid). The cropping area of most farms however lies under this figure according to the survey.

Because of the greater power force expended during the work peaks to date a displacement of work to other periods of the year occurs for animal traction. Thereby, regionally varying modified work peaks could be created for non-mechanized work operations.

1.2 Labour distribution

Depending upon the structure of a farm-household system the available labour force, the investment capacity and the incentive to mechanize can vary substantially. Mechanization can reduce the load or increase the burden of men, women and children to a varying extent. Since in general only a certain number of working hours can be achieved by the farms, changes in the time investment for the different work operations can have a direct impact on the remaining work. According to Kirk (1987) in North Cameroon the work peaks can shift from seedbed preparation to harvest by the exploitation of all possibilities of mechanization with animal traction.

One part of this work can be accomplished by increasing the burden on the family members (e.g. women and children for weeding) or transfer to seasonal labourers, depending upon the kind of distribution of labour in the family. Family labour forces should be used to the maximum. (Persoons, 1988) It is only worthwhile to do other work with draft animals and to invest in new implements in addition. This applies especially for seeding, which generally is a work peak only in areas having short vegetation periods. At the same time, however it requires a high investment for equipment and places demands upon the technical level in terms of utilization, maintenance and repair.

As long as some work operations have not been mechanized the expansion of cropped areas will continue to lead to an increased mobilization of hired labour from outside the farm. Animal traction increases the overall demand for labour forces, especially seasonally (Kirk, 1987). Rural exodus and land distribution are the primary factors which influence the supply of labour forces. In South America a class of landless rural farm workers is being created due to the extreme concentration of land ownership and releasing of labour forces in the course of motor mechanization; this differs from the situation in Africa.

In Africa animal traction is particularly being introduced by larger families who can afford the investment and additional expenditure arising from the expansion of cropped areas or animal husbandry. The increasing importance attached to formal education leads however to the phenomenon that child labour, e.g. for tending the animals, competes with school attendance.

The division of labour based on gender is highly influenced by ethnic and religious backgrounds. Some ethnic groups do not allow the women to participate in agriculture. Generally, the women in Islamic regions, for example, the Toucouleur, Fulbe and Haussa south of the Sahara are seldom seen working in agriculture, which leads to an increased demand for labour forces from outside and favours the advancement of mechanization. (Valentin and Spittler, 1976; Kirk, 1987)

In general, draft-animal work is carried out by the men. Women and children do manual work such as weeding. The problem exists that animal traction can be an advantage to the men, in that by the undertaking of tasks such as transport or expansion of cropped areas at the cost of women's fields they attain a better economic position. Credits for teams and equipment are mostly directed to the men.

In many regions, particularly in Africa, the women have their own fields and the profits do not always flow into the family budget. In a project in the Northwest province of Cameroon it was attempted by the introduction of animal traction to make the work on the women's fields easier and to integrate the men into cropping of fields, since the importance of their traditional activities (hunting, harvesting tree crops) was receding. The project was exclusively directed to the men, who became owners of oxen with their own fields by way of a loan contract. The position of women was adversely affected since -their workload increased due to non-mechanized tasks with the expansion of cropped areas, - the traditional women's fields were reduced, leading to a decline in the income of the women and their diminishing social standing.

Following the outbreak of protest from the women, who saw themselves being reduced to mere farmhands, the programme was extended also to include the women. Nevertheless, the additional workload and the minimal amount of animal use remains a problem on the women's fields which are difficult to mechanize (mound cropping).(Bruchhaus, 1984; Zweier, 1986)

Many farms are run solely by women, especially in the areas bordering South Africa. In Botswana and Lesotho this applies to 40 - 50 % of the farms. Tasks done with draft animals are conducted by the women; only plowing remains a matter for the men.

In South Brazil the distribution of tasks is influenced by the European immigrants:

- The men work with implements on the fields. Storage, transportation and marketing are also their domain.

- Women on the whole work more. They take care of the household, the garden and the animals. Weeding with the hoe and harvesting are also among their main tasks.

- Seldom does a woman operate draft-animal implements or the tractor.

- On the other hand, men are more often observed to be daily wage earners in weeding and harvesting, especially in coffee and cotton growing regions.

Mechanization can therefore have an impact on the intra-family division of labour, create a demand for labour forces outside the family and lead to a supply of services for draft animals.

1.3 Crops

Only certain crops are suited for mechanization, which merely plays a subordinate role for permanent cropping. Only weeding is carried out with draft animals in some permanent crops, e.g. coffee in Brazil. Some tuberous plants such as yam are difficult to mechanize. A balanced working calender for the Centrale region, additionally favoured by the tropical rainy climate, renders the use of implements superfluous, while the working calender in the drier Savanes region (figure E 5) shows up defined work peaks (see section F 2, case study on Togo).

Most annual crops, on the other hand, are easy to mechanize. Crops which are broadcasted such as wheat or barley only require the draft animals for plowing and in some cases for harrowing. Draft-animal implements facilitate especially seeding and weeding of crops such as maize, sorghum, beans, cotton or groundnuts, where row cropping is combined with the demand of a wide spacing. Weeding is especially important for plants that possess the trait of a slow initial development. Crops which are traditionally connected with ridging can easily be mechanized. The ridger is also partially used for both soil preparation and weed control.

Since the introduction of animal traction is today frequently done in conjunction with growing certain cash crops, e.g. labour-intensive cotton, a considerable surplus investment can occur with harvesting. As a consequence of modified work peaks due to the expansion of areas under cash crops there is less time available for work operations for other crops under some circumstances. The cropping schedule therefore changes because of mechanization. Thereby, typical crops can become prevalent according to the respective level of mechanization. For example, in South Brazil beans which are predominantly harvested by hand are primarily grown on smallholdings that work with animal traction, whereas it is more advantageous to produce wheat and soybeans on larger motor mechanized farms due to the possibility of mechanizing the harvest.

Mechanization with draft animals has an impact on mixed cropping systems. In these systems there is an interaction between the different plant types, for a minimum of two crops, at least during a part of the vegetation period. They build up a system similar to the natural vegetation forms, as opposed to pure stands with only one plant type. Particularly in traditional cropping systems aimed at sustainability, numerous stable, very complex mixed cropping systems have evolved. In contrast to pure stands they have the advantage with otherwise identical conditions that they yield a larger and more certain harvest on the same area, exploit the soil more efficiently and provide a more complete soil cover (erosion control, low weed invasion, low evaporation). (MSnn, 1986; Steiner, 1984; Vieira, 1985)

Sanchez (in: Andrews and Kassam, 1977; modified), distinguishes, aside from crop rotation systems, between the following mixed cropping methods:

- mixed intercropping: mixed crops without any regular arrangement, mechanization can only occur for soil preparation

- row intercropping: two crops simultaneous or sown in rows in very close succession (e.g. maize and beans either in separate rows or alternately in the same row); well suited for mechanization with soil preparation, seeding with animal-drawn implements can be rendered more difficult due to protracted seeding dates, closer spacing causes problems for weeding

- strip intercropping: two crops in narrow plots beside each other; no problems for mechanization

- relay intercropping: staggered planting within one vegetation period with overlapping of partner crops; soil preparation and seeding of second crop is difficult

- multi-storey cropping: association of small annuals and higher perennials; unproblematic with sufficient vertical and horizontal spacing.

Mechanization by means of animal traction requires in part a modification of these cropping systems, since the plots can no longer be uniformly cultivated and tilled. As long as only the seedbed preparation is mechanized mixed intercropping systems can be maintained without any regular arrangement if the seeding times of the included crops are not too far apart. In Botswana with mixed cropping of maize, sorghum, cowpea and other plants broadcasting is practiced and the seed is subsequently plowed in with draft animals (N 1989). On the other hand, mound cropping, as with the women's fields in Northwest Cameroon, can not be easily mechanized.

The introduction of animal traction is connected with a reduction of such cropping systems and, in general, a transition to row cropping occurring in parallel. The simultaneous planting of two different crops could be done with seeders by initially sowing every second row. In Brazil, an animal-drawn implement has been designed to sow beans and maize together (Mantovani, 1986). It is more difficult to seed if, for example, maize is sown two weeks after beans, or if beans are sown in a nearly mature maize stand. In order to avoid damage to the neighbouring crop seeding is usually done manually. A very efficient hand-operated seeder is being used in Brazil (see section G 2: case study Paran Weeding becomes difficult if the row spacing is too small and one plant type has already developed further. In this case a single-blade cultivator is employed, which however often causes damage to the roots of the crops, or weeding is done by hand hoe. (compare Erbach and Lovely, 1977)

In part, the share of mixed cropping is being reduced in order to achieve better work with the cultivator. Also, chemical fertilizer and herbicides are increasingly being applied, although it is very difficult to proportion the dressing in concert with the needs of the adjacent plant. This constraint ultimately triggers the gradual disappearance of mixed cropping systems.

1.4 Work operations

In the survey the proportion of work done with draft animals in the individual work operations -clearing, seedbed preparation, seeding, weeding and transportation - on the farms was investigated. The percentages total to 100 %, so that changes in the emphasis on singular operations have direct consequences for the share of other work operations. Since no draft animals are normally used for harvesting in the regions examined (only for transporting the crop), a shifting of labour investment for the individual work operations, e.g. with greater cropping area, cannot completely be determined in the survey.

On the average, soil preparation with draft animals takes up the largest share, followed by transportation, weed control and seeding (figure E 6). Clearing and harvesting as well as other work operations (such as threshing) are not significant for animal traction according to the survey. The figures must be attributed in part to soil preparation or transportation.

Soil preparation (plowing, cultivating, ridging) is without exception in all cases a component part of draft-animal work. The regions where direct seeding is practised and no soil preparation takes place are very limited.

If the investment for soil preparation decreases due to the introduction of draft animals and the cropping area is expanded, then work bottlenecks arise with weeding and harvesting. Persoons (1988) reported on Mali that a farmer can plant 2.5 ha of cotton, but can only weed 1.5 ha and harvest 0.8 ha. To date the harvest (except for groundnuts) has been left out of draft-animal mechanization. The mechanized work operations in Europe and North America such as clearing of tubers or mowing of grain are only mechanized on the motorized level in highly technicized regions such as South Brazil. Work bottlenecks during harvest operations are all the greater

- the further advanced the mechanization of other work operations on the farm are
- the more the biological-technical modernization on the farms is applied
- the more the harvest is tied to deadlines (Kirk, 1987).

With full exploitation of animal traction for the work operations the harvest in North Cameroon, for example, requires 56 % of the labour investment occurring during the vegetation period (Kirk, 1987). Often the previous work operations are not mechanized because of bottlenecks arising during harvesting. According to the survey only in one-third of the regions is seeding conducted by draft-animal seeders. Animal-drawn seeders are used to a greater extent in areas where agriculture is characterized by

- permanent cropping
- tradition and greater distribution of draft-animal husbandry.

Partially an already high proportion of motor mechanization is achieved. This however does not apply for the West African countries. The soil is worked thoroughly; the mouldboard plow and often the harrow are used, however in no case the ard plow.

Seeding with animal traction plays a role in the survey wherever the number of humid months is above 4.5. In tropical wet climates with more than 9.5 humid months the work operation is again exclusively carried out manually.

Brazil is an exception here (e.g. the state of Sao Paulo, for which no questionnaires were distributed). Exactly in this climatic zone draft animals are intensively used for all work operations. The high land-use intensity, the tradition of animal traction by European immigrants and the industrialization play a particular role here. Also, in the immediate neighbouring state of Paranwhich has a constant wet and hot summer subtropical climate, there is a high proportion of work achieved by animal-drawn seeders.

According to our experience seeders are utilized in practice on the farms where animal traction is intensive and

- the technical level has reached an overall high niveau, such as in Brazil,

- money flows into the farms from wages earned elsewhere and labour forces are scarce, such as in Lesotho, where 50 % of the men work in South Africa,

- the vegetation period is very short and the crops must be sown as quickly as possible, such as in Senegal and Mali.

More than half the cases in the survey in which seeders are utilized are located in Brazil. In the Andes and most African countries seeding is not conducted with animal-drawn implements. The use of seeders represents, in the technical and economic view, the highest level of development of animal traction in Africa and South America.

Although the responses originated essentially from regions with animal traction, weeding however is done exclusively manually in more than 30 % of the instances. Thus, the relatively low proportion, where weeding with draft animals is found, also in regions having a tradition of draft animals, is surprising, although the labour productivity can be considerably be increased thereby. With greater land-use intensity the effort for weeding increases over-proportionally and leads to a steep reduction of labour productivity in the hand-hoe system, up to a point of limitation of the total production of a farm. Simultaneously, where the R value is high this work operation can be more easily mechanized with draft animals. In his analysis of animal traction in Cameroon, Kirk (1987) has determined that weeding is only gradually being mechanized. This work operation is precisely the area in which the highest labour savings can be achieved.

In the regions where no weed control occurs with draft animals, where soil preparation and transportation are the primary draft-animal tasks, the work is carried out over-proportionally at the manual level (upto 78 %). One of the reasons can be sufficient available labour forces on the farm. Further reasons for not mechanizing weed control could be that

- investments and repairs (abrasion) could present a risk where the infrastructure (procurement of spare parts) is poorly developed

- seeding would have to be done in exact rows,

- work operations would be less tied to schedules,

- in many cases the work would not have to be carried out by the farm manager.

Considering the high labour investment for weed control the decision remains for the farmers, when a scarcity of labour forces exist, either to limit the cropping area and to conduct intensive weeding or to take the risk of poor weeding on larger areas. The latter choice would be connected with greater investment for soil preparation and additional costs for seed.

Weed control with animal-drawn implements occupies a significant proportion of the work operations in all climatic zones (table E 2). Due to the risk of weed invasion great importance is attached to it in the tropical wet climates. On the other hand, in tropical dry climates where weed invasion is not severe it can be easily accomplished.

Where the ard plow is commonly employed, weed control is done with this implement when animals are engaged for the task.

Sowing with seeders only takes place where weeding is carried out with draft animals. However, it was mentioned only half as many times by the respondents in the survey. Sowing with seeders also increases directly proportional to the increasing share of weed contol in the work operations conducted with draft animals. Both work operations gain importance with increasing distribution rates of animal traction within all draft-animal activities.

With a distribution of animal traction in less than 5 % of the farms, the seeder is practically not represented, while weed control already shows a mentionable share of the draft-animal tasks. Seeding with animal-drawn implements first plays role with a greater distribution.

With an expanding cropping area the share of seeding increases only slightly (figure E 8). The high figure for farm size of 10 - 20 ha is attributed to the South Brazilian cases, where motor mechanization is prevalent and the draft animals are still only employed to a minimal extent for soil preparation. The importance of weed control is continuously receiving more attention; this means that for larger cropping areas it will become a bottleneck.

For all plot sizes the share of seeding remains under 2 %; only in the 0.6 -1 ha range does it increase to 7.5 %. Weeding operations increase with the size of the plots.

The possibility of transportation with on-farm draft animals is minimal in one-quarter of the cases or not exploited at all (17 cases under 10 %). This applies particularly for the Andes countries and Ethiopia, where the ard plow is used but usually no other animal-drawn implements. In all cases animal traction has a tradition and is widespread. Here, animal traction appears to remain on the level of own implement fabrication by the farmers themselves. It must be taken into consideration though, that the ard plow can be employed as multipurpose implement, for breaking furrows during seeding and weed control.

With less land-use intensity transportation becomes the most important work operation. Thereby, animal traction has a below average distribution (16.3 % share of the total work) and is partially still in the initial phases. Regarding the heavier work operations, as in plowing, the use of the tractor is gaining popularity, also on draft-animal farms. In this case the harvest residues simply are worked in, the soil is better prepared and is easier to work in subsequent operations with other implements. The tractor can however only function under suitable conditions (deep soils, slight slopes, few obstacles). Its use considerably increases the risk of erosion. With the transition to motor mechanization the emphasis of the individual work operations employing animal traction is being shifted

With an increasing use of tractors the soil preparation with draft animals decreases by more than half. The farms that do not immediately switch over to motor mechanization hire the tractors for soil preparation, while the following work operations are conducted by draft animals (mixed mechanization). Especially seeding gains importance, whereby the better technological environment and the greater land-use intensity play a significant role. Most of the reported cases are in South Brazil (5 out of 6). Experience in Africa confirms this tendency (table E 4). Transportation with draft animals retains its importance, also where a higher proportion of motor mechanization is found.

In the draft-animal regions motor mechanization is initially employed for soil preparation in all the cases. Also, transportation has a prominent standing. The subsequent work operations are only represented to a slight extent, whereby seeding is mentioned most frequently. This data correlates with the experience reported, where seeding follows soil preparation in mixed mechanization with a tractor.

The interaction between the individual work operations, the mechanization level, the cropping system and natural endowment can be well illustrated with the case of Lesotho. The great majority of farms own an animal-drawn plow Due to the use of the tractor the share of draft-animal plows is lower in the lowlands and foothills. animal-drawn seeders and cultivators are primarily employed here, as row crops such as maize, sorghum and beans take up the greatest part of the cropping area. In the mountains, on the other hand, wheat and barley are mainly broadcasted. Tractors can not be used here because of an unsuitable topography.

The substantial number of seeders is attributed to the limited time span for soil preparation and seeding due to the early frost, the savings from wages earned in South Africa and the scarcity of labour forces. The marketed share is therefore very low; only 19 % of the farms have any surplus produce to sell. (Groann, 1986)

2. General features of the implements

2.1 Overview of implements

The descriptions of the implements are subdivided according to the following categories: soil preparation, sowing and application of fertilizer, weed control and harvesting. A distinction is made between soil preparation and seedbed preparation only when necessary. Mechanical weed control, also representing a kind of soil preparation, is treated separately, although the same implements are partially used for soil preparation and weed control, such as the ard, ridger and chisel plow or cultivator.

For further classification of soil-preparation implements two approaches are possible: according to design or to manner of operation.

For this treatise it appears appropriate to index the implements used for soil preparation according to manner of operation, since they are directly connected to the requirements of the respective location. Design is employed as a further criterion, but it is subordinate to the manner of operation. One can thus distinguish between:

- implements that operate symmetrically to the line of draft power, as the ard,
- implements that work assymmetrically, as the mouldboard plow,
- rotating implements such as the disk plow or disk harrow.

Implements that work symmetrically to the line of draft power and rotating implements mainly loosen and mix the soil, while the mouldboard plow primarily turns it. In the group of symmetrically working implements most belong to the category for soil preparation. These are:

- ard,
- ridger,
- chisel plow and cultivator,
- shovel-type implements as the fuor.

Implements for both soil preparation and weed control are collectively considered under the term chisel plow. In the narrower sense the term chisel plow is used for deeper soil preparation with tines and superficial tilling with duckfoot, sweep shares, etc. (e.g. bico de pato; section G 2.4.4).

If the implements are employed for weed control, then the term cultivator is used. The fuor is treated in section G 2.4.2. Implements for harvesting are hardly used in rainfed cropping in the regions investigated. Merely the groundnut lifter plays a greater role (section F 3.4.5). The use of mowing implements for grain harvesting are conceivable. However, the introduction of such implements, which has been attempted in Senegal and Brazil, has to date been unsuccessful. Under the conditions in South Brazil, with a high technological level and the promotion of wheat cropping, this implement might be worthwhile for the farmers. It could increase labour productivity by a factor of 20. However, it is questionable whether the expected number of sales could be an incentive for the farm machinery manufacturers. Only then could they be purchased for an appropriate price. (Fabry, 1990)

Multipurpose toolbars are used in numerous quantities in West Africa, especially as chisel plows and cultivators. The larger equipment such as the Ariana (figure F 20) or the similarly designed Policultor 600 in Brazil have been distributed under the auspices of special development programmes or in training centres; there are only a limited number of them, however. The sales of the wheeled tool carrier (Polyculteur in West Africa or Policultor 1500 in Brazil) have been less encouraging (for the reasons see section F 3.4.7 and Starkey, 1988a). The policultors were manufactured by CEEMAG, but production has been discontinued. At present they are fabricated by APAC.

Harvesting implements and multipurpose toolbars are not treated in great detail here, but receive attention in the case studies.

2.2 Design and maintenance problems

Under tropical or subtropcial conditions as for example in South Brazil the implements must often work on fields having large quantities of organic matter (growth of fallow, weeds or harvest residues). The most frequent constraints in these regions are working in this mass and the resultingclogging.

A low weight of the implements is of importance where the plots are far apart or are located on slopes (e.g. at various altitude levels because of the distribution of risk) and the associated transport of the plow to the fields.

The transport of implements to the fields can be done in different ways:

- the farmer carries the implement (figure E 11),
- it can be loaded on the animal, the cart or a sled (also forked branch),
- it can be dragged.

Dragging frequently causes damage to the implement, also the handle can become bent. This leads to a imbalanced burden on the farmer during the work. Sometimes, the implement remains on the field, which can lead to rapid deterioration and theft of the parts.

The handles generally cannot be adjusted to the tallness of the farmer. This often leads to a unbalanced bodily load. In one region differences of the height of the seeder handles were found to be between 89 and 114 cm and the widths between 51 and 76 cm (Casao et al., 1987).

A key problem of the technical functioning of the implements is the bearings for rotating parts. Abrasion of the wheel bearing on mouldboard plows is frequently reported. A one-sided wheel mounting, as with the implements in Togo, brings negative results. Simple repairs done by farmers, e.g. replacing the axle with water pipes or wooden bars, become very difficult. A fork-type mounting is then recommended. Occasionally, the wheel has too small a diameter for the soil characteristics or the existing plant growth. Solid wheels and too small wheels clog easily in wet and heavy soils. Large-dimensioned spoke wheels would be more appropriate in this case.

The publications often suggest the use of a supporting skid as an alternative, but in practice they are seldom encountered.

Connections with bolts are also a risk factor regarding potential damage. Prior to the introduction of mouldboard plows the work was done manually or with traditional animal-drawn implements; now a new unknown technology has been introduced whose principles of functioning are not simply understood.

Moreover, suited tools are lacking (spanners etc.) and often the incorrect sizes or the wrong parts are delivered (Togo, Tanzania, Zimbabwe, Niger). Worn out parts then cannot be exchanged, rendering the work difficult. Therefore, the adjustment of seed covering scrapers, tines of cultivators or the regulation of fertilizer applicators should be designed to be altered without the use of spanners. In order to avoid a loss of bolts some joints should preferably be welded. The reason given by Kenyan farmers for the disappearance of bolts was "screw-eating dogs", which was one of the grounds for the failure of multipurpose implements (Neunher, 1984). The loss of a clamp bolt leads to the fixing of the working width of a cultivator with wooden wedges, as shown in an emergency repair of an adjustment in South Brazil

Seeders place the highest demands on manufacturing and maintenance in the regions investigated, because of the required precision for the many rotating parts.

Most of the implements are treated in the following sections, although some are discussed exclusively or in greater detail in conjunction with case studies because of their regional importance.

3. Field preparation

3.1 Requirements

A central problem for the utilization of implements, especially in the humid climatic zones, is the huge amount of organic material lying on the fields at the start of the cropping cycle. Fallow, both short and winter fallow that has a high weed infestation, or undecomposed harvest residues are the cause of this condition. This frequently leads to the practice of burning. Thereby not only nutrients but especially the organic matter content is reduced, which the cation exchange capacity and the stability of the aggregate maintains. To prepare the fields for subsequent soil preparation and further work operations the following processes are necessary, depending upon the climate and land-use intensity:

- clearing and removal of tree stumps and roots,
- chopping of vegetation from short fallow,
- management of harvest residues,
- working in or mulching of green manure.

In order to reduce the workload and to prevent burning animal-drawn implements are partially utilized. However, to date very few techniques exist at the level of animal traction to clear the fields.

3.2 Implements

Simple implements have been developed for the removal of tree stumps. For mulching weed material or green manure crops, especially in humid areas, only mowing bars, which to date have not been accepted in the regions investigated, and knife rollers exist, which are also suitable for processing harvest residues. The disc harrow (section E 5.3) can also be used for working in harvest residues. In South Brazil the knife roller is commonly found, both on motor-mechanized farms which practice no-tillage as well as on smallholdings with draft animals. Further experience has been made in Northeast Brazil, Tanzania and Cameroon, where they are however not widespread.

The knife roller consists of a roller made of wood or metal, upon which flat metal knives are mounted on the circumference. The principle of operation is that the knives of the roller bend over or chop off the stems of plants as it passes over them. The expended pressure depends upon the number of knives and their spacing, which in turn is determined by the circumference. If the number of knives is increased with the same circumference, then the pressure is reduced and the risk of clogging in the space between the knives increases. Further factors determining the efficiency of the roller are: working weight, construction material for the knives and mounting angle of the knives. The quality of the work is determined by the fiber and moisture content of the plants.

In South Brazil to date the knife roller has primarily been used to chop residues of fallow. In the meantime, it is also recommended for the mulching of green manure and chopping of harvest residues (maize stalks). However, it cannot process all types of green manure, for example black oats. On slopes of over 20 % and on stony ground the knife roller can hardly by utilized. David (1988) states that the speed of oxen is too slow. This implement is manufactured by the farmers
or by local artisans.

Trials have been made at IAPAR to improve the knife roller. The roller material (tree trunks, oil drums), working weight, method of applying weights, number of knives, circumference and cutting angle of the knives were tested. In the TIRDEP project in Tanzania the knife roller has also been tested for weed control, especially after fallow and in terrain having many roots. The roller proved to be essentially well suited for work in grass up to 3 m high following fallow and for weed control in permanent crops. Usually two working runs were sufficient.

For weed control after seeding, which would be feasible with a smaller implement in principle, the implement was less appropriate. With weed control in maize the spacing between the knives remained too high, also at a maximum number of knives, so that the weeds could not be destroyed at an early stage of growth. Furthermore, during this season the wet soil clogged the knives. For draft-animal use one model made of wood with assymetric application of draft power has proven to be suitable; it is arranged so that the animals need not walk directly through high plant growth. For the protection of the animals, especially on slopes, either the roller is covered or the attachment of a drawbar is necessary. The advantages with the drawbar are improved maneuverability, smaller headland (6 m instead of 10 m with chain) and easier reversing in the case of obstacles (Becker, 1987).

A knife roller developed in Northeast Brazil that weighed 70 kg proved to be too light. The recommended weight should be between 250 and 400 kg, depending upon the draft animals and the existing vegetation. It is advantageous to be able to adjust the weight. However, no moveable fill material should be used, e.g. water. This would reduce the quality of the work, leave tracks and cause an extra load on the animals. As an optimal solution Becker (1987) investigated an implement in Tanzania of 1 m diameter with 80 cm-long knives and 8 -10 knives made of tempered and sharpened steel -used leaf springs can be applied for this purpose - with a spacing of 25 cm at an mounting angle of 0 degrees. A sub-divided or flexible knife roller would be a more useful implement, but this would raise the cost considerably. (Figueiredo, 1988; Becker, 1987; compare Bertol and Wagner, 1987) The labour saved with a 3-year growth of grass (organic mass = 10 t/ha) as opposed to manual work is substantial: with the knife roller 6 days/ha vs. 70 days/ha by hand. (Becker, 1987).

4. Soil preparation

4.1 Soil fertility

All cropping measures must be directed to the conservation of soil fertility. Soil fertility is defined as the natural and sustainable potential of the soil with respect to the production of crops (Klapp, 1967). The fertility of the soil is decisively influenced by the soil-preparation methods undertaken. The following physical, chemical and biological components determine the soil fertility.

The texture, i.e. the relative proportion of fine and coarse particles present, provides information on and leads to conclusions regarding the pore distribution, structure stability and nutrient supply. Soils with a high silt content can store the most amount of moisture available to the plants. Sandy soils usually hold little moisture for the plants, as they cannot counteract the forces of gravity. They do not generally possess a stable structure, since the surface forces of the sand grains are minimal. Thus, the organic components of water-storage capacity and the structure of the soil are decisive. Loamy and clayey soils indicate generally stable structures, since the greater inner surface area leads to stronger attractive forces between the soil particles. As also clay minerals are the carriers of cation exchange capacity, the natural fertility of these soils is better than sandy soils. (Dehn, 1981)

The soil structure is the conglomeration of various soil particles in aggregates as well as their shape and arrangement. It determines the distribution of coarse, medium and fine pores that affect moisture availability and drainage. This is extremely important for precipitation conditions in the tropics. There, the soil structure, namely the building up of aggregates, is created by swelling and shrinking, root growth, activity of larger soil fauna and soil tillage. The soil structure depends, among other things, upon the proportion of iron and aluminium oxides and the types of clay minerals.

Severely weathered soils have a high percentage of iron oxides and kaolinite, which has the property that it does not swell with exposure to moisture and subsequent drying. Less weathered soils are characterized by a high share of minerals that are able to swell. The proportion of coarse pores is significant for infiltration and the exchange of gases. In tropical rainforest infiltration rates of several hundred mm/h are reached due to the high proportion of pore volume, so that even with intensive rainfall there is no surface runoff (Sanchez, 1976). The coarse pores also determine the space in which roots can potentially grow.

The bulk density is determined by the share of pore volume as well as the relationship of mineral to organic matter. For optimal root growth the soil should be loose. Bulk densities of between 1.0 and 1.2 g/cm3 are reasonable. The compaction of a loose soil by 0.15 g/cm3 can already reduce root growth to about half (Trouse, 1979: in Dehn, 1981). Thus, the achievable moisture potential is reduced for the plant, which is crucial in zones having high rainfall fluctuations. Particularly critical for plant growth are abrupt density changes (such as clay concentration horizons, plow sole), which also can lead to a perched water table.

Plants often react more sensitively towards soil than air temperature. When the soil is protected from direct sunlight the soil temperature corresponds essentially to the air temperature in the humid tropics. Without cover this can rise to more than 15°C higher. Soil temperatures of over 35°C approach the upper limitation for plant growth.

Building up a high stable content of humus by supplementing the soil with organic material and hindering a too rapid decomposition must be the priorities of a sustainable agriculture. Humus and organic matter can decisively improve the properties of the soil. Nevertheless, the practice of removing organic matter and residues, for example by burning or the pasturing animals, is common. Measures for supplying organic material, such as green manure and application of animal dung, are not being exploited in most of the regions investigated.

4.2 Soils

In most cases the regions in the survey do not possess similar types of topsoils, rather soil associations are encountered that are influenced by a host of factors; here the landscape relief plays a prominent role.

The properties of the components of the associations are important for soil preparation. Moisture and clay content of a soil determine the soil stability and thus the tillability. The optimal range for tillage is very narrow for clayey soils. The more clay a soil contains and the drier it is, the harder is its condition. The space for roots, one of the most important aspects for plant growth, is limited in shallow soils.

Furthermore, the moisture supply for the plants in the thin soil layer is not always assured due to low moisture storage capacity. This factor is especially important where short dry periods also occur during the vegetation period. The risk of erosion is critical, since the thin arable layer can rapidly be removed in contrast to soils having a greater depth. Shallow, stony soils are difficult to till. Oxisols (USST), the most severely weathered of all soil formations, are predominantly found on relatively flat, old land surfaces. They are very deep and usually have a stable structure, and are very suited for mechanized cropping. Despite the high clay content (upto 80 %) they often occur as loam or loamy sand because of the building up of stable micro-aggregates in the fields. The bulk density is very low, so that in part with compaction (e.g. in tractor tracks) higher yields are achieved due to a better moisture supply. Two days after a heavy rainfall the soil can be tilled.

Most possess few nutrients except for those originating from volcanic primary rock. On slopes, from which weathering products are constantly being removed by means of water erosion and soil flow, Ultisols and Alfisols (USST) occur as recent formations. They have a somewhat higher natural soil fertility, but are structurally less stable. In part they are characterized by greater texture differences between the A and B horizon, so that there is a severe risk of erosion.

Moreover, gravel deposits or stone layers can limit the tillability near the soil surface on the upper slopes. During dry seasons the Alfisols become very hard, rendering soil preparation impossible. These so-called "millet soils" in West Africa tend to form crusts and to possess a higher bulk density, which makes root growth difficult (Klaij and Serafini, 1988).

Inceptisols, a classification of newer soil formation, occur where the soil removal process has reached hardpan. Relatively fertile soils can be created on freshly weathered hardpan and in the sedimentation basin of rivers, if the sediment did not originate from Oxisols from the older highlands. Vertisols (USST) are nutrient-rich lower lying soils having a high clay content, which have originated from basalt or are created on a stowage level in depressions beside older soils (reformation of clay minerals), where however nutrient deficits can occur, e.g. phosphorous and potassium. A pre-condition for the creation of Vertisols is a changing wet climate, in which they are subject to high moisture fluctuations and regular drying out. Vertisols represent an extreme case here, due to their high clay content and the high proportion of swelling clay minerals. The most suitable range for tillability between too wet and too dry conditions is very narrow (minute soils).

They can therefore not be optimally tilled. Sandy and silty soils having a low structural stability tend to form a sealed surface, crust immediately and therefore undergo risk of erosion. The breaking up of crust formations can increase infiltration and thus reduce the surface water runoff, the trigger for water erosion; the impact of this measure for weakly structured soils is rapidly reduced, especially with rainfall. Sand achieves a high bulk density of 1.5 g/cm3, and under heaviest compaction upto 1.7 g/cm3. The compactions are solid, and thus no roots can penetrate them. This is most evident with fine sand, which has the densest compactions. Drier sand can be tilled; the measure may be useless however since it does not retain its structure produced by the tillage operation. (Roth, 1989)

Aside from stones in the narrow sense, laterite concretions can render the soil preparation difficult. In a semihumid/semiarid climate iron-rich amorphic mass (Plinthite layer) can occur deeper in the soil at the break-off point on edges of slopes, which can arrive on the surface by tillage and dry out irreversibly (Sol Ferralitique RemaniFS). In soils which have often been tilled pea-size concretions are found, which can take up to 50 % of the soil profile, e.g. in the humid tropics of West Africa.

4.3 Toposequence and soil types

The soils change along a slope with regard to depth and clay content. These changes can occur within a few hundred meters, depending upon the topography

In the humid tropics steep slopes are seldom found. Wavy, hilly landscape without rugged edges (half-oranges) occur or flat land, such as found in the Congo and Amazon basins. Stones are rare. On slopes there are soil sequences, e.g Oxisol, Ultisol, Inceptisol (USST). In the wet and dry climates of the humid tropics (e.g. South Brazil) their are more jagged slopes. In savanna climates the sequence can consist of Alfisol, Ultisol and Vertisol (USST) ("le rouge, le gris et le noir"). (Roth, 1989) On the upper part of the slope the soil can be flat and stony. The risk of erosion is high due to the inclination and the shallow soils allow no margin for soil loss. The clay content, the depth and the water storage capacity increase at lower levels. The soils on the upper slopes are correspondingly easier to till, also manually. Alluvial soils, heavy black soils having a substantial quantity of organic matter, are found in valley bottoms. Because of the soil moisture they can be used year round as pasture (e.g. Vertisols in the valley bottom with a changing wet climate, such as in Zambia, Malawi, Tanzania or Ethiopia). They require high investment of energy; in part, they can only be cultivated after considerable expenditures for water management and drainage. On the other hand, the risk of drought is greatest on the upper slopes. The farmer must weigh the lower power input requirement against the greater risk of drought. This risk decreases with increasing humidity.

(Pingali et al., 1987)

The zone preferred for cropping depends upon the climate and the population density. In arid areas the lower slopes or valley bottoms receive preference. In semiarid areas cropping begins on middle slopes and replaces the pastures on the lower slopes and valley bottoms with increasing population pressure. In humid regions the upper slopes are also cultivated. Labour-intensive water management measures are only worthwhile in lowlands when sufficient labour resources are available on the basis of the population development.

The intensification of soil preparation on the medium slopes leads to severe erosion problems for many tropical soils. Due to the heavy soils in the valleys the transition from the hand hoe to the plow takes place here first, according to Pingali et al. (1987). This does however not apply generally, as the example of Casamance (Senegal) shows, where animal traction is utilized more on the plateau. Also in south Paranhe plow becomes more widespread on the upper slopes.

4.4 Objectives

Various aims are pursued with soil-preparation measures:

- weed control, especially prior to sowing,

- creation of a certain surface structures (e.g. ridges); seedbed preparation for smooth operation of seeders; crumbling of soil for special crops; preparation for irrigation,

- loosening of poorly structured, tightly compacted soils; creation of coarse pores for better root penetration,

- working in of organic material or chemical fertilizers,-increasing the infiltration by means of loosening soil, especially breaking of crust,

- reduction of evaporation by destroying capillary structure or hindering growth (full fallow).

In general, loosening only serves a purpose when the soils have previously become compacted, e.g. by heavy tractors, implements or animals. Further aims such as bringing leached soil components to the surface are of lesser importance with the shallow working depth of draft-animal implements.

4.5 Various aspects of soil preparation

4.5.1 Impact of utilizing implements

The mechanization of soil preparation alone does not produce a quality gain in comparison to the hand hoe, and thus does not improve the area performance (yield per ha) (Pingali et al., 1987). Weed control with the hand hoe, also a soil-preparation measure, is considerably more effective. With draft-animal use beside the increase of labour productivity only the possibility of cultivating unused heavier soils is given.

The mouldboard plow has become widely distributed in the tropics and subtropics at the level of animal traction, in contrast to motor mechanization where disk implements dominate. Its decisive advantage is an effective weed control. It leaves a finer seedbed than the ard or chisel plow. Frequently, the implement, which is adapted to cropping in temperate climates, has been introduced by European settlers in new agro-ecological zones. The implements used for the subsequent work operations are designed for work on well prepared fields following plowing.

Less intensive preparation with the chisel plow or the ard are particularly widespread in the semihumid/semiarid regions. The soil is loosened without turning. In some soils, e.g. Vertisols in Ethiopia, the ard is the only implement used for soil preparation. Further work operations can hardly be carried out due to an unsuitable soil structure or clogging. Access to the wet, poorly drained fields is very difficult, for example for weed control.

Contradictory investigations have been apparently conducted on the advantages and disadvantages of soil preparation, especially with the plow. These deviating statements can be attributed to the considerably differing basic conditions of soil type and climate, however. Yield increases after plowing (Charreau, 1974: in Pingali et al., 1987) and a reduction of erosion have been determined (Charreau, 1972 in: Sanchez, 1976) in semihumid/semiarid regions having soils that tend to become compacted, while in humid regions less significant yield growth (Vincente-Chandler, 1966 in: Sanchez, 1976) and an increase of erosion has been measured (Marquez and Bertoni, 1961 in: Sanchez, 1976).

Plowing causes a temporary reduction of soil bulk density. The enlargement of pore volume however does not apply to all pore size classifications. Plowing creates essentially large pores favouring root growth, especially important on soils having a higher bulk density and non-swelling clay minerals (kaolinite). Thereby an increase of the infiltration rate is achieved, at least for a certain period of time. The medium and fine pores determining the moisture content capacity can only be created biologically or physically (swelling and shrinking), and can be destroyed by working the soil.

A disadvantage is that by intensive soil preparation, especially with the mouldboard plow, the soil is more intensively aerated and warmed, the decomposition of organic matter is accelerated and moisture loss causes higher evaporation. Plowing means, in addition, an over-loosening: the loosened structure is not initially suited for cropping and it takes time for restabilization of the soil. Mechanical loosening by means of soil preparation possesses only limited stability. After a sort time the bulk density can already be greater than for no-tillage and in the long term it can be higher than the latter (Armon and Lal, 1979 in: Dehn, 1981). The looser the soil is after tillage, the more sensitive it is to compaction. This applies especially for a sandy soil having little organic matter.

After a some recompaction higher moisture capacity will is achieved. Many soils become depleted with prolonged cultivation. Due to compaction of the topsoil when uncovered or the creation of compaction horizons (e.g. plow sole) they become less permeable and more susceptible to surface water runoff and soil loss. Intensive soil preparation, especially the establishment of a fine-crumbed structure, contributes to a reduction of infiltration due to a decline of aggregate stability and surface sealing. Water drainage can take place unhindered if the surface is uncovered. No resistance is provided against wind erosion. A coarse seedbed preparation, as for example with the ard, therefore brings with it a reduction of risk against erosion. Smallholder agriculture also contributes to damage caused by erosion, particularly due to the penetration of hilly terrain (figure E 18).

The individual crops have an varying impact on the amount of soil loss; the following ranking have been determined for humid regions (table E 5).


Soil loss %

Crop %

Soil loss

Maize-beans-mixed crop










Sugar cane








Sweet potato




In the various cropping regions the sequence is adjusted to concur with the seeding date, since the impact of erosion tends to vary in the course of the year.

Soil preparation is minimized or totally omitted for no-tillage under mulch cover. The soil is covered with organic material. The no-tillage method is referred to when no soil preparation has been carried out over several vegetation periods. Minimum tillage or the no-till approach are less suited for soils that tend to become crusted or compacted, are poorly drained or undergo little biological activity (Hartmans and Kuile, 1983). Weed control remains a constraint for no-tillage under wetter tropical conditions.

4.5.2 Soil preparation in semihumid/semiarid climates

Tillage at the beginning of the cropping period in the wet season In dry regions (dry savanna, semi-desert) agriculture is at risk due to a scarcity of water. Here, a humus-conserving, water-saving soil preparation is critical. Turning the soil leads to a loss of moisture. Therefore, minimum soil tillage with the chisel plow or no-till methods are applied, followed by a breaking of the soil capillarity during weed control. Traditionally, ards are often used for this purpose. The use of the plow is not recommendable in these zones due to the risk of erosion (wind, water) and the low area performance. The organic matter required for mulching is difficult to produce here, since the cropping of green manures, for example, is not possible because of the scarcity of moisture. Harvest residues are usually no longer available to cover the soils, as they are necessary for animal fodder.

Fieldwork is generally begun after the first rains. In Morocco, for example, the ard is utilized for surface tillage. Due to the short duration of the growth period in many regions in West Africa and Northeast Brazil soil preparation is only carried out on the surface and directly after sowing or no-till operations. Sowing must take place as rapidly as possible after the first rains, otherwise the yield declines drastically. No-tillage is favoured by the occurrence of sandy soils and low risk of weeds in the Sudan zone of Africa. Working in of organic matter at this point becomes superfluous (figure C 7). In order to perform plowing in the wet season the first rains must moisturize the soil to a sufficient depth. Subsequently, 4 to 5 days of work (25 hours) are necessary to plow one hectare with a team of oxen (Bordet et al., 1988.)

Where soils tend towards compaction, as in most of Senegal, soil preparation could be more favourable than soil-conserving no-tillage to achieve a better root penetration and thus a higher yield. Simultaneously, the infiltration could be improved. Studies in dry regions showed an increase in yield by means of soil preparation; rice improved the most while groundnuts the least: sequence -rice, sorghum, maize, cotton, millet, groundnut. (Charreau, 1974: in Pingali et al., 1987)

Other trials in sandy Alfisols in Senegal have proved the positive effect of superficial soil preparation with the hand hoe as well as deep plowing, in comparison to no-tillage (Nicou, 1972 in: Sanchez, 1976). In this case there was no difference in yields between the fields cultivated manually and those with the tractor. Considering an economic assessment the result would be an increase of profits in West Africa, particularly for cotton, rice, groundnut and maize (in this order). (Pingali et al., 1987)

Although various studies (e.g. Chopart, 1981) have proved the positive effect of plowing on the yield and these findings have become priority areas for the extension services, plowing is not accepted in some regions, e.g. in Senegal. Plowing with animal traction is only beneficial in rainfed cropping in the Sudano-Sahel zone when precipitation is above 900 mm per annum. This statement must be modified corresponding to the type of soil and practices accompanying plowing: The heavier the soils and the higher the moisture uptake or retention capacity, the more recommendable is plowing. (Bordet et al., 1988)

In order to overcome the limitation of tillage because of the short vegetation period, the time of soil preparation could then also be selected at the end of the vegetation period or during the dry season. Both of the two methods would be suitable for increasing the water uptake during the first rains of the rainy season. The procedures are discussed further below.

Tillage at the end of the cycle or during the dry season Soil preparation at the end of the cycle, e.g. with the chisel plow and rigid tines, would loosen the soil and thus increase the infiltration during the first rainfalls. At the same time, harvest residues could be worked in. This method, as recommended by research, has proved to be impracticable.

The following reasons speak against this approach:

- It requires a repetition of soil preparation for seedbed preparation at the beginning of the rainy season with the above described negative effects, and thus means an extra work operation.

- It competes with the harvesting operations and requires the removal of harvest residues grazed throughout the dry season.

- The agronomic effect is disputed, since the effect is possibly no longer evident by the time the rainy season begins (Bordet et al., 1988).

A further useful method is soil preparation during the dry season with the chisel plow. Various tines have been developed in the Sahel zone for soil preparation where precipitation is under 900 mm (figure E 19). (Bordet et al., 1988; Sene, 1988)

The work in dry seasons is only possible on very sandy soils. But even for numerous light soils in Senegal this is not possible because of the required high draft power due to compaction. The first operations showed that the necessary draft power overloaded the oxen teams. A further developed tine, which was pulled by 2 oxen of 400 kg in good condition, required a draft power of ca. 90 kp in light soil (clay content of 12 -15 %). The working depth was 9 cm; the infiltration profile was deeper than without tillage (Le Thiec and Bordet, 1988). However, the operations did not go beyond the bounds of the experimental station.

4.5.3 Soil preparation in transitional zones

Cropping on ridges is widespread in transitional regions of the semihumid/semiarid climate, e.g. in Casamance in south Senegal (1000 -1300 mm rainfall) and in the Savanes region of Togo (1000 - 1100 mm rainfall). This is practised primarily in Africa (88 % of the cases) according to our survey; in South America it is often used for some crops (potatoes, tobacco). To a great extent the ridger is used exclusively for preparing these fields. Ridged cropping offers, aside from its application in irrigation systems, particular advantages in the regulation of the moisture supply:

- With suddenly occurring high quantities of rainfall in this generally drier region plants stand above water and ridged soil drains well.

- Ridged cropping reduces the surface runoff and increases infiltration. Therefore, storage of water in the deeper layers is greater than cropping on flat soil.

- The soil is only partially tilled, and narrow unworked strips remain under the ridges.

- The ridged soil is loose, favouring the growth and harvesting of tubers and groundnut.

- In cold mountainous climates the ridges offer protection against light frost due to their influence on the microclimate.

The increase of water storage is particularly important for many of the semihumid/semiarid-occurring Alfisols and Ultisols, whose storage capacity is low. Ridged cropping has advantages if the dry season sets in at a later part of the growth period and the roots have penetrated to a deeper level. The crops are protected against waterlogging caused by heavy rainfall. During dry periods in later growth phases the plants can protect larger water reserves in lower layers. By shifting the ridges for the subsequent crop an efficient weed control is achieved. (Dehn, 1981) To control evaporation a compacted, smooth or a loose surface of the ridges is desirable, depending upon the climate and the soils.

Cropping on ridges promotes a more rapid mineralization. Frequently the harvest residues are placed into the furrows, the ridges are flattened, covering the residues. In Senegal (south of Sine-Saloum) methods are used to rebuild ridges by cutting perpendicularly to the old ones. This facilitates soil preparation when low amounts of precipitation occur at the beginning of the rainy season. Ridged structures provide protection against erosion, as long as the rainfall is not so great that it causes the ridges to burst on hilly terrain. A system of tied ridging (figure E 20) has been developed to reduce soil losses, which can be substantially greater than on flat seedbeds. Wind erosion also is reduced by cropping on ridges (Fryrear, 1984: in Klaij and Sarafini, 1988).

According to Bouchet, director of SEMA in Boulel, Senegal (cited by Gaudefroy-Demonbynes, 1957: in Bordet et al., 1988) cropping on ridges increases yields by upto 20 % where high precipitation occurs (more than 1000 mm per annum). In these wet areas more time is available for soil preparation and weed control presents more serious problems. According to our survey ridged cropping however is also frequently practised where low average precipitation occurs (between 500 and 1000 mm).

4.5.4 Soil preparation in humid climates

In the wetter regions usually only the migrants from the savanna zones practise cropping on ridges, e.g. in the Centrale region of Togo (1200 - 1300 mm rainfall). This leads to the conclusion that the ridges originated from the transition from semihumid to subhumid climate, where due to the high humidity already a greater importance is attached to weed control than in the dry savanna. On the other hand, cropping on mounds is widespread in humid climates, especially where the land is used less intensively. The topsoil is accumulated in mounds and thus nutrients are collected. The cropping area on mounds is small. Weed control plays a lesser role in this system and tree stumps are not an obstacle.

Covering the soil represents an essential measure for conserving soil fertility in the tropics and subtropics. According to Rockwood and Lal (1974) the main advantage of mulching, in combination with minimal soil tillage or no tillage, lies in the assured and cheap reduction of erosion. The effect of the mulch consists in protection from the impact of raindrops, which causes surface sealing. This advantage has an effect especially in regions as e.g. in South Brazil where 60 mm per hour or 250 mm per day at seeding time are not unusual. Here, an effective erosion control is only assured by means of a permanent soil covering (Derpsch et al., 1988).

Soil fertility is influenced positively by means of no-tillage under mulch cover whereby soil temperature fluctuations are reduced and higher temperatures are avoided. A slower mineralization occurs due to the low cultivation intensity (Lal, 1975). A higher moisture availability is achieved by a reduction of evaporation and higher infiltration rates in no-tillage under mulch cover. The biological soil activity is increased with prolonged application of mulching under the no-till method.

Despite the positive impact of no-tillage under mulch cover reported by many authors (e.g. for South Brazil: Monegat, 1985; Derpsch et al., 1988) this method in the humid regions is not widespread in practice in terms of animal traction, especially in Africa. The no-till technique places high demands on the management and the cropping of green manure for the production of necessary mulch means an extra investment. Significant problems such as weed control without extra inputs as herbicides, as well as nutrient dynamics remain unsolved. Appropriate draft-animal implements for sowing on unprepared soil is not yet ready to be put into practice.

In contrast to the semihumid/semiarid regions weed control and the working in of organic matter represent the main constrain in humid areas. More time is available for soil preparation due to the longer vegetation period. Therefore, soil preparation with the plow predominates here on the level of animal traction.

5. Implements for soil preparation

5.1 Symmetrically operating implements

5.1.1 Ard Designs, manner of operation and distribution

Soil preparation with ard plows of many various types is appreciably widespread world-wide. According to Schultz-Klinken (1981) approximately 75 % of the farmers in North Africa, Southeast Europe, the Near and Far East, and Latin America work with this type of implement. The models are distinguished by the material from which the plows are manufactured, wood or metal, and their design.

Depending upon the specific design ards have three basically different functions:

- breaking a furrow and leaving a ridge on both sides, and partially turning the soil,
- breaking a furrow and leaving a ridge on one side, and partially turning the soil,
- loosening the soil in layers.

The various designs of the point, frequently clad with an iron reinforcement, yield either a more breaking, digging or cutting effect (Schultz-Klinken, 1981). The ard plow is known for its superficial and efficient operation. The chief characteristic function of the ard plow is soil preparation which does not turn the soil and may leave some unworked patches. Should a totally worked plot be necessary for the subsequent tillage, then several work operations in criss-cross fashion must be carried out. Because of its particular design the ard does not leave a clean field with the first attempt. This need not be a disadvantage. Unworked patches and a rough surface prevent wind and water erosion (Hopfen, 1969). Ards are directly connected to the yoke by means of a the long drawbeam and usually have a single wooden handle. Since they have no support wheel, the working depth can only be regulated by the pressure expended by the farmer. This depth is 5 -15 cm for simple ards and 15 - 20 cm for further developed implements and metal ards. According to the survey the working width is between 5 and 25 cm, independent thereof whether a simple or more improved version is being employed. The weight of the implement also varies, but is generally under 30 kg since the ard must be carried to the fields by the farmers.

Most ard plows can be built by village artisans or by the farmers themselves. Thus, they are of low cost and repairs present no insurmountable problems. However, only certain types of wood can be used due to stability requirements. Hopfen (1969) distinguishes initially between two basic types of ards: the body ard and the beam ard. As for the body ard the plow beam and the handle are constructed of one piece and the drawbeam is directly attached to this component (figure E 21).

The beam ard usually has a curved wooden drawbar through which a working tool, for example a peg or the plow beam, is pierced. The handle is connected separately to the plowbody or drawbar. Variations which are specific to certain countries or regions have evolved from these two basic types. The sole ard is an example for a third type. Body ards are the most commonly found of all ard plows. They are sturdy implements and have a relatively great working depth. They have a shoe-type share and are used in soils having sufficient moisture content. Body ards are found in the Mediterranean region, Asia and in some Latin American countries, especially Peru.

The beam ard is probably the oldest form of ard plow and has a limited working depth. It is primarily used for surface working of the soil and has a kind of prong used as a tool for drier, stony soils, and occasionally a slip- shoe type share for heavy soils. The design of the beam ard has several problematic points. It has a narrow plow body with a point-shaped share tip, which is exposed to the soil resistance without any further supporting brace. Due to the concentration of soil resistance on this small surface the beam ard is difficult to manoeuvre and to keep in balance. Furthermore, strong draft forces are placed on the contact point between the plowbody and the drawbar. By means of the insertion of a connecting link made of leather or wood the draft force is better distributed and absorbed. Because of these weak points this type of ard plow is best used on soils having no obstacles such as roots and tree stumps. This implement is chiefly found in the Mediterranean region as well as eastern India.

The sole ard works the surface of the soil and is easy to manoeuvre. It has a long, flat plow body, upon which the handle and the drawbeam are fastened. Due to its shallow layer-wise loosening operation it is well suited for drier areas. Deep plowing is not possible because of the long sole. Sole ards are found in the Mediterranean region, Afghanistan, Pakistan and Nepal. The first two plow types ridge and mix the soil on both sides of the furrow; this leads to considerable moisture loss.

Because of the manner of operation ard plows are less popular in wet climates. Krause et al. (1984) consider this implement to be more suited for drier conditions in comparison to the mouldboard plow, since it reduces expansion of the soil surface area that is exposed to wind and water. The large number of various ard plow and share types demonstrates that with these implement variations adapted solutions can be found for locally occurring problems. Further developments should receive appropriate attention.

In the survey in 85 cases ard plows were mentioned 15 times regarding soil preparation. In comparison to the world-wide significance of the ard plow this is only a small proportion; the reason lies in the non-representative execution of the survey. The occurrence of the ard plow is recorded for Bolivia, Ecuador, Peru and Ethiopia.

In Bolivia and Ecuador the beam ard is used. In spite of the suitability of the body ard for wetter regions (Hopfen, 1969), the maresha is widely distributed in Ethiopia; it is considered to be a type of the beam ard (figure E 22).

It is utilized in Vertisols, which are subject to severe expansion and shrinkage due to the high proportion of clay. In the survey it was reported however that the maresha is well suited to heavy soils, since other implements would require a greater power input. When wet, the soil sticks to all the metal parts, increasing the weight and causing a poor work result due to smearing. An advantage is the narrow working width of 5 cm, which offers the soil only a small surface for sticking.

Ard plows in Latin American countries have a greater working width of 10 - 25 cm. According to the survey the area performance in the Andes countries is approximately 30 - 40 h/ha and in Ethiopia according to Starkey (1989) 40 - 50 h/ha, whereby several working operations are necessary. Experience

Regions where ard plows are utilized demonstrate a relatively high quotient of draft animal use. In half the cases animal traction is common on more than 50 % of the farms, although regional differences are evident within the countries.

The primary application is for soil preparation. Weed control and breaking furrows for seeding are also carried out with the ard plow. The seed is placed by hand directly into the furrow that has been dug by the ard and is immediately covered by a second movement (figure E 23).

94 % of the draft animals used for pulling ards are oxen teams. Horses, donkeys and mules are seldom employed, except for weeding or transportation. According to the survey the regions where the ard plow is primarily used are all above an altitude of 1000 m. The soils are heavy, mainly in Ethiopia, and medium soils are found in Latin American countries.

Wherever the ard plow is widely distributed there is also a high land-use intensity. In three-quarters of the responses permanent cropping is almost always conducted and obstacles are hardly evident. On the other hand, in 44 % of the cases stony ground exists.

The cropping area on all the farms lies under 5 ha; more than half farm less than 2 ha. Tendencially, the Latin American countries have the largest arable area. The plots are very small (in 66 % of the responses under 0.4 ha). Cropping on ridges is very uncommon in ard-plow regions. In the Andes countries modifications of wooden ard plows are employed, all having a relatively great similarity with each other (arado combinado, arado andino) (figure E 24).

They are designed according to the traditional concept, however they consist of metal except for the drawbar. Various tools can be mounted (ridger, mouldboard, cultivator tines), so that their use is multifunctional.

Improved models have gained poor access to practical situations and their application is limited to development projects. Several reasons however could speak for their utilization. Meier (1987) reported from the highlands of Peru that the wooden plow represents a commandable technique, but it breaks quicker and appropriate wood is scarce. The metal plow is not heavier than the traditional implement so that the farmer can carry it to the field. In addition, the improved ard is easier to pull, has a greater working depth (13 - 15 cm) and can work the soil in two operations, as opposed to the normal three runs.

A significant hindrance to their dissemination is the high price, approximately four times that of the traditional wooden plow. Since the farmers in ard plow regions predominantly grow crops for their own subsistence (Gryseels et al., 1984; Meier, 1987), they are not able to pay for them and must resort to manufacturing their own implements.

5.1.2 Ridger Manner of operation

The ridger does not turn the soil completely. It leaves ridges on the surface of the soil, and does not work the soil under the ridges (figures E 25 and E 26).

Usually, the old ridges are plowed through and thus broken up prior to the subsequent cropping period. Another approach is to plow diagonally to the previous ridges. If the plow is adjustable the ridges can have a variety of forms: gentle or steep slope, narrow or broad ridges.

The shape of the ridger body has an impact on the soil and the draft power requirements. Ridger bodies having a broad share tip, a smooth body edge and steeply set high wings (figure E 27) split up the soil in wedges and leave a well rounded furrow. The bottom of the furrow and the side of the ridge thereby acquire an undesired compaction and the smooth, compressed surface dries more quickly, leading to a loss of moisture.

Flatter ridger bodies having a sawtooth-like body edge (figure E 28) can avoid creating compaction and allow a surplus of loose soil to pass over the mouldboard, which spreads over the furrow and the ridge, thus yielding a protective cover. Moreover, the draft power requirement is less for this shape. (Franz, 1969)

Ridgers can be constructed as swing plows (figure E 25) or as single-wheel plows, whereby the wheel increases the stability for guiding the implement and also makes the work easier. Distribution and experience

For many of the countries in the survey the ridger may be characterized as a universal implement. Ridgers are often the only animal-drawn implement of African farmers. They are used for working operations in seedbed preparation, ridging as well as weed control. A comparison of the distribution of the ridger and mouldboard plows showed that in 58 % of the regions where the mouldboard plow is employed the ridger also exists. It has scarcely been accepted in ard-plow regions such as the Andes countries and Ethiopia. Here, its use is limited to development projects or special requirements, such as irrigation crops in Peru and the Dominican Republic.

The traditionally widespread practice of ridging in some of the regions in the countries of Togo, Senegal, Zambia, Ghana, Malawi, Mali and Burkino Faso make the ridger to one of the most frequently used implements. Its rapid and superficial manner of working the soil is highly prized. Contrary to the recommendations of extension services, which propagate the prior preparation with a mouldboard plow, the ridger is often used directly for seedbed preparation. In the Savanes region in North Togo, for example, the time for a more intensive seedbed preparation is too short at the beginning of the rainy season.

In contrast to the mouldboard plow the ridger achieves approximately double the area performance (Nelles, 1989; Viebig, 1982), because of the greater working width and since it only works half the field. However, with a corresponding working width the draft power requirement is higher. A disadvantage for direct building of ridges is that the vegetation under the ridges is not disposed of and weed infestation can rapidly occur. Therefore, the effect of weed control with the ridger for soil preparation is generally not as useful as the mouldboard plow.

In wetter regions with a longer vegetation period, for example Bobo Dioulasso in Burkino Faso, the mouldboard plow is used first, followed then by ridging operations.

The effectivity of the ridger against weeds is not very highly estimated; particularly high plants cannot be easily destroyed simply by covering them with earth. The survey showed however that in half the responses the ridger was in fact employed for weed control. This is frequently the case in all countries with the exception of the Andes countries, Brazil and Zambia, where the ridger is not or seldom used for this work operation.

If work is carried out with the ridger, the crop rows are easy to identify in fields with high weed invasion. In Togo the ridger is used in combination with the hand hoe since sufficient labour forces are available; after weeding the implement is then employed to build up the ridges. The survey showed that generally there is a higher land-use intensity in regions where the ridger is common than in mouldboard-plow regions. Problems with obstacles were not mentioned by the respondents, with one exception in Chad. This can be attributed to the fact that the soil had already been worked with a mouldboard plow.

The workmanship and material quality is in general assessed as good. Because of the symmetrical direction of draft power the handling is less complicated than with the mouldboard plow. The risk of error with the adjustments is lower. The width adjustment is placed directly on the plow body, where it is better protected against damage. In contrast to the mouldboard plow the point of attachment is more stable, since it is not connected with the width adjustment. In general, an uneven abrasion is counteracted by the symmetrical distribution of force. In Togo it was ascertained in a survey that the parts exposed to abrasion wore out slower on the ridger than on the mouldboard plow.

5.1.3 Chisel plow Manner of operation

This section deals with chisel plows and cultivators having rigid and semi-spring tines. They may be identical with implements used for weed control. For soil preparation however narrower tools are more frequently employed; thus the implement has a mode of operation similar to that of the chisel plow. The chisel plow works the surface of the soil and creates a loose structure on the top soil layer without turning it. The vertical tine breaks the compacted horizon by simply pushing the soil towards the front and then to the side. Some unworked strips remain between the furrows, but smaller voids are created than with plowing. (Preuschen, 1951) Simultaneously, a separation occurs, transporting small crumbs downwards and larger clods to the surface (Segler, 1956). This can be beneficial to the soil, since the covering with clods prevents evaporation during dry periods and also counteracts surface sealing and erosion caused by hefty rainfalls.

Various tools can be connected to the tines. Chisel shares, semi-and full duckfoot and sweep shares are commonly found in practice. A narrower tool such as the chisel share creates a nearly triangular furrow profile in the soil by pushing the compacted horizon. With increasing soil moisture the profile becomes narrower and the soil is broken up to a lesser extent. When broader duckfoot or sweep shares are employed a trapezium-shaped profile is created (Gill et al., 1968 in: Wieneke and Friedrich, 1983).

The wider the tool, the greater the draft power requirement. If the chisel plow consists of only one tool, its function becomes similar to that of the ard (figure E 29 and E 30). The bico de pato and the fuor belong to this category of implements, as discussed in the case study on Brazil (see section G 2.4).

In most cases designs incorporating several tools are concerned, which usually belong to the category of multifunctional implements. The following types showing various designs are distinguished:

- a main frame (Houe Occidental, Senegal <see figure F 16>; Peco-tool, Sierra Leone; Houe Manga; Niger, Burkino Faso),

- T-shaped frame (Houe Sine, Senegal <see figure F 17>; Arara, Senegal <see figure F 18>; CEMAG Policultor 300, Brazil),

- triangular frame (Houe Triangle, Burkino Faso; Togo <see figure F 7>; Planet cultivator by Sans or Tatu <see figure E 62>),

- rectangular frame with two wheels (Ariana, Senegal <see figure F 20>; CEMAG Policultor 600, Brazil <see figure E 40>).

Three or five tools can be mounted on one implement. The working depth can be adjusted by the wheel, if used, and the depth is normally 3 -5 cm (Metzger, 1988). The chisel plow can achieve a better area performance than the plow. In Senegal the plow required 25 h/ha, whereas the chisel plow only needed 5 h/ha (Metzger, 1988). On hard soils the draft power requirement can be considerable (Preuschen, 1951). According to Starkey (1989) a chisel plow equipped with three tines requires the same draft power as an 8" conventional plow having a working depth of 20 cm. Thus, in most cases three tools are utilized, as also was ascertained by the survey. Distribution and experience

Chisel plows are used as soil-preparation implements individually or in combination with the plow. They also serve the purposes of seedbed preparation as well as weed control shortly prior to seeding. Chisel plows are less widely distributed than the plow in the countries surveyed. They are more frequently found in countries such as Senegal, Mali, Niger, Burkino Faso and a few of the regions in Northeast Brazil in semiarid areas having 2.5 -5 wet months. Precipitation is normally between 500 and 700 mm.

Soil property is the main determinant for the use of the chisel plow. This factor was most often mentioned as a reason for its mobilization or as a constraint. This implement is preferred for light, sandy soils where its manner of functioning has proved to be advantageous against risk of erosion.

Frequently, a shallow and rapid soil preparation is desired. In regions where direct drilling is widespread, for example in Niger and Senegal, the chisel plow is utilized for soil preparation. The land however must be essentially free of weeds and vegetation. Since mention is hardly made of clogging, one can assume that the soil surface is free of obstacles and is uncovered. This was stated to be the reason for the use of the chisel plow in one of the regions of Mali. Since the R values lie between 50 and 60 in the chisel-plow regions, the low occurrence of obstacles is not attributed to land-use intensity, but rather to the dry climate and the sparse vegetation (figure E 31).

Ashburner and Yabilan (1988) report of trials for using different implements in Niger. The effect of various implement types on the yield of millet was examined on an experimental station and in the field. Tests were carried out with the mouldboard plow, various types of chisel plows as well as the ridger. The experiments were conducted on sandy soil in the Departement of Tahoua, where traditional ly the no-tillage method is employed.

The trials demonstrated that the mouldboard plow increased the yield, however the working speed of 19 h/ha is very slow and the implement is not adapted to this region because of erosion. The ridger also requires relatively much time, so that it becomes difficult to seed on the same day. In order to be able to use to ridger, the soil must first be worked with a chisel plow. Soil preparation can be much more rapidly carried out with a chisel plow. The area performance with three tines was 9 h/ha for the Arara chisel plow and the Houe Manga. Although the Arara is well adapted to some regions it proved to be too heavy for these soils. In addition, the design was assessed to be too complicated. The Houe Manga is lighter, but has design deficiencies in the spring tines, thus hampering its operation.

The advantages of working the soil with a chisel plow are best appreciated in sandy soils having a proportion of clay that tends to build a soil crust. A test with various tools attached to the chisel plow (duckfoot, bar-point share) did not bring any significant yield increase. These relationships are also presently being investigated by the ICRISAT Sahalien Centre in Niger. Overall, it was determined that a significant yield increase could be achieved by soil preparation with the plow and chisel plow and direct subsequent broadcasting, in contrast to traditional, direct drilling without prior soil reparation. It is salient however to apply a rapid method of working the soil, since under these climatic conditions it is more important to minimize the risks than to maximize the yield. For this reason and due to the soil-conserving effect the chisel plow is more adapted to this location.

In heavy soils the utilization of the chisel plow shows up some problems. This was reported for some regions in Ghana, Brazil and Niger. Here the draft power requirement increased considerably (Starkey, 1989) and higher weed growth can no longer be mastered with the chisel plow. Therefore, according to Tchougoune (1988) for heavy soils in valley bottoms and depressions the use of the chisel plow is more advantageous.

In regions where the building up of ridges is widespread, the use of the chisel plow is also not accepted. In Togo the farmers reject the idea of an additional work operation. The implement does not fit into the cropping system in Ghana. The working and material quality of the chisel plow varies considerably. Houe Occidental, Houe Sine and the Arara have a good reputation. With some of the other implements the critical point is the tine attachment. On the contact point between the flexible and rigid part it has been observed that the tines easily break off. The tines also come loose. The width adjustment is not always easy to manage. Finally, severe abrasion can be caused by inexact assembly.

5.1.4 Harrow Manner of operation

The harrow works at a comparably shallow depth. The clods are broken up by diagonally placed tines or spikes and thrown to the side. As with the chisel plow, the harrow creates a separation effect. In order to accomplish a successful job the working speed must be high. Therefore, the draft animals used are often horses or mules.

The harrow is used to brake up a crust soil surface, for crumbling coarse clods and fine seedbed preparation. It loosens and aerates the soil. It can also serve the purpose of working in the seed after broadcasting, levelling and weed control.

Numerous designs of harrows exist and different tine structures. The tools may be either rigid or spring tines having a variety of dimensions. The frame is manufactured from wood or metal. The most simple design is the single-section, triangular harrow with rigid tines (figure E 32). A further design is the single-section, rectangular harrow (figure E 33). Harrows are available in single-and double-section designs. The latter can easier adapt to uneven soil surfaces. In general, harrows can be characterized as simple in design and ease of handling. Working width and depth are not adjustable. Distribution and experience

Generally, the harrow is seldom used in the surveyed regions. In the few differentiated answers all above mentioned types were represented.

The harrow is used primarily in Brazil and on farms in a few African countries (Mali, Zimbabwe, Lesotho, Chad). The concentration in regions having a subtropical humid climate such as Brazil and the semiarid climate in African countries (figure E 35) are rather remarkable. Permanent cropping (R value 80 -100) exists in almost all cases. Only in two regions in Mali and Brazil is the land-use intensity low (R value 33 and 42, respectively). Problems of clogging are hardly mentioned. This occurs in regions where the harrow is seldom found in practice and in project areas.

Wherever the harrow is frequently used, seeders are also employed. This was determined for African countries, with the exception of Chad, and also for Brazil. Unfortunately, very little experience and reasons for using the harrow in Brazil has been reported to date, although it was mentioned most (12 out of 18) here. Its widespread use can be attributed to both European influence and the frequent subsequent application of seeders. For some crops and cropping methods the harrow is also advantageous, even if no seeder is employed. This is the case when rice or other grain types are broadcasted. By means of a subsequent run with the harrow the seed is rapidly worked in just below the surface. The use of the harrow in connection with growing rice was reported for Mali, Cameroon and Chad.

On the whole however the harrow is little used. In only 42 % of the instances where the implement is known is it actually used (18 out of 43 cases). This means that over half of the farmers reject the harrow as a soil- preparation implement for various reasons. Ecological reasons remain the main grounds for a negative assessment. The harrow removes organic material from the soil and hinders a mulching effect. The heaps of residues left from cleaning the harrow tempt the farmers to set them on fire (Gutsche, 1989). The remaining litter however is of great importance for the conservation of soil fertility in the tropics and subtropics. In addition, apprehension exists regarding the increase of erosion due to the use of the harrow.

Further frequently mentioned reasons for not utilizing the harrow are the growing of crops on ridges, the occurrence of obstacles and the unsuitable topography. On heavy wet soil it functions poorly, since the tines clog and smear the earth instead of breaking the clods. On light soils the harrow can be substituted by a simple dragged device for levelling, which need not be transported from field to field. (Gutsche, 1989) Finally, an additional working operation which appears superfluous to the farmers finds no acceptance.

5.2 Assymmetrically operating implements

5.2.1 Mouldboard plow Manner of operation

The mouldboard plow turns the soil by cutting a furrow slice and depositing it to one side partially overturned. A coarse loosening occurs thereby and a mixing and crumbling of the soil; the volume increases in the process. An expansion of the surface area can lead to greater evaporation rates and a more rapid decomposition of the humus. Therefore, it is hardly suited for arid areas due to the sensitivity of the moisture supply and only applicable to certain conditions in the humid tropics because of the rapid decomposition of the organic matter (Krause et al., 1984; Viebig, 1982). On the other hand, the operation of the mouldboard plow facilitates the working in of plant material, harvest residues and manure in the soil. In wetter areas with a high weed growth at the beginning of the rainy season this is conducive to seedbed preparation.

The plow body and often also the frame of the mouldboard plow is made of metal. The mouldboard and the share are designed according to the required use. The flatter the slope of these two parts, the easier it is to pull the plow. The crumb formation is however poorer and the tendency of sticking increases with clayey soils. (Preuschen, 1951) The greater the tendency of clod dispersion in the soil, the shorter and steeper the plow body should be shaped. Otherwise the danger exists that the soil does not slide along the entire length of the mouldboard but falls from the mouldboard too soon and hampers the turning process.

The tendency of clod dispersion is less in heavy soils and the breaking of grassland. (Eichhorn 1985) Basically, steep share shapes crumble better and require greater draft power, while with extended curved shapes the deflection above the share is less and little crumbling is achieved. (K1930; Segler, 1956; Dencker, 1961) In principle the following mouldboard forms can be distinguished: cylindrical, cylindrical-helicoidal (as a medium shape) and helicoidal (CNEEMA, 1981). (figure E 36)

Their application is as follows:

- steep cylindrical shape: for light soils (loose sandy soils; as a steep, short shape for light soils that tend to be sticky),

- partly cylindrical, sinusoidal shape: for medium soils, for sandy loam or loamy sands (universal shape),

- helicoidal, flat ascending shape with a pointed cutting angle: for heavy, overgrown cohesive soils, crumbles less.

Starkey (1989) differentiates for Africa between the short, cylindrical shapes suited for rapid tilling in light soils and semi-helicoidal shapes for high weed infestation in humid climates, which cause a less abrupt inversion. Similarly, there are different share shapes. On hard overgrown and stony soils it is frequently difficult to penetrate the top layer, especially with a worn share tip. Beak-type shares are more suited for such conditions than the normal shares. According to the survey the normal shares (2/3 of all instances) are more widely found than the beak-type share.

The shares are generally manufactured from steel which can be forged out and tempered in rural workshops with the aid of a simple open hearth and the usual dipping in water. They are reinforced at the cutting edge, so that the appropriate material is available for reworking the correct share shape. Shares made of hard cast iron to withstand a greater amount of abrasion, can be constructed to self-sharpen and are cheaper to fabricate; but they cannot be employed on stony soils because of their brittleness and cannot be sharpened by means of forging. Lateral and share pitch provide for the entry of the plow into the soil, especially for hard soils (figure E 37). They change their shape when worn and must be reworked by forging. (Dencker, 1961; Estler et al., 1984; K1930; Matthies, 1987) An adaptation of the furrow width to the draft power of the animals can be accomplished by regulating the working width. This is only possible to a limited extent, since a certain relationship must be maintained between working depth and width (1:1.2 to 1.4), if the quality of the work (turning, crumbling) is not to be hampered. If the plow body works at a constant depth, the risk of plow sole compaction arises. Designs

Four types of plow are commonly found: swing plow without a wheel, a single-wheel plow, gallows plow with two wheels or the frame plow. Conventional mouldboard plows are generally not connected to the yoke with a drawbar but rather are pulled by a chain.

Swing plows are light, maneuverable and can be purchased for a reasonable price. The working depth cannot be adjusted, and thus the penetration of the share depends on the soil resistance. The results are irregular. For soils that are difficult to till fluctuations of upto 50 % are recorded, meaning a variation of between 5 and 15 cm for a target depth of 10 cm (Preuschen, 1951). The regulation is controlled by the farmer, so that the work is physically very strenuous.

With the single-wheel plow the vertical movements are kept to a minimum by the furrow wheel; therefore it is possible to adjust the working depth. When the working width is adjusted a lateral pressure is exerted on the wheel and the bearing by means of the transverse forces. Good lateral and depth control is maintained with the gallows plow, which is equipped with a double- wheel forecarriage. Depending upon the model it is also possible to operate it in a self-controlled mode, so that the farmer need not adjust the handles.

The frame plow (figure E 38) is a further development, which has a frame instead of a leg and is suited for the attachment of several plow bodies. All occurring forces can be supported by the wheels. Often there is a third support wheel at the back. The working depth is adjusted by means of the wheels. This permits total foolproof operation for working depth and width. (Preuschen, 1951) Multipurpose implements such as the Policultor 600 in Brazil or the Ariana from West and South Africa may come under this classification. Because of the self-control mode frame plows make the work easier, however on small plots and steep slopes they are difficult to maneuver due to their considerable weight.

Gallow plows and frame plows are expensive compared to swing plows and single-wheel plows and are preferably used on flat, well cleared land. In principle mouldboard plows can be designed as conventional plows or reversible plows. Conventional plows Conventional plows are equipped with one plow body, which only turns the soil to one side (figure E 39). For return runs through the field separate furrows are necessary. In order that the distance between the two furrows does not become too long, the larger fields are divided into plots (e.g. with a width of about 1/3 the length of the field). These can be plowed by the casting or gathering method. If the plowing is done by gathering the turning circle becomes increasingly narrower to the point where the animals must walk over plowed ground. The conventional plow leaves ridges or a furrow within the field. A continuation of this procedure over the years can cause the soil to be eventually transported out of the field by the casting method, for example, as was reported in the survey. Conventional plows are also unsuited for work on slopes, since the slice of earth falls downhill from the one side of the plot. Its use is recommended only for slight slopes. In comparison to the reversible plow the conventional plow is lighter, easier to handle and cheaper. Its advantage is the minimal problem of clogging, since the low point of gravity also allows a relatively high frame. In general, conventional plows achieve a greater working width and low specific resistance than reversible plows due to the suitable design of the plow body. The working width may be adjusted by altering the point of attachment.

Reversible plows

Reversible plows have plow bodies that can turn the soil to both sides. They are designed as two-way turnover (figure E 40) or turnwrest plows (figures E 45 and G 21). The two-way turnover plow consists of a right and a left inverting plow body; both can be designed to serve their respective purpose in an optimal manner. The turnwrest plow is equipped with a symmetrical plow body, which can be turned on a bolt hinged on the body under the leg. The plow body can not always be designed to accommodate all field uses, since the symmetrical mouldboard represents a compromise and its working width and depth are limited by the height of the leg. An advantage is the low point of gravity in comparison to the two-way turnover plow and the low weight of the turnwrest plow as well as the low purchasing cost. The reversible plow allows the working of a furrow to the same side. Thereby the disadvantages of tilling with conventional plows are avoided and an even field surface is created. The path along the headland to the next furrow is saved and thus a lesser turning time. This is a particular advantage where the turning diameters are small. Reversible plows are therefore especially suited for small irregular plots. Headlands must be somewhat greater when the reversible plow is used. On slopes plowing must be done so that the soil is thrown to the uphill side to counteract erosion. This is only possible with the reversible plow. According to Franz (1969) the uphill turning of the soil can be done on slopes upto 25 %.

Reversible plows often have a low frame height due to the higher centre of gravity; this can easily lead to clogging. In addition, dirt in the retainer can lead to a delay in engaging the plow body. Distribution and applications of mouldboard plows

In two-thirds of the cases where mouldboard plows were mentioned in the survey they are actually used for agricultural purposes. In at least one-quarter of the cases they are used exclusively in development projects or are very seldom applied under practical conditions. Aside from a few pilot projects this is the case wherever other implements such as the ridging plow or the ard are common. In the Savanes region in Togo cropping is done on ridges. For this purpose the ridging plow is employed without prior plowing. In Senegal in the Sine-Saloum region there is not sufficient time for seedbed preparation with a plow, and thus the no-till method is applied.

Obstacles and steep slopes are the most frequently mentioned constraints and this hinders the work with a mouldboard plow. Instead, non-turning implements are used there, which are easier to employ and less problematic under these conditions; also they are less expensive. However, they leave a significantly more inhomogenous seedbed, so that the use of subsequent implements is rendered more difficult or becomes impossible. The occurrence of stones is low in mouldboard-plow areas. The cropping area of farms having mouldboard plows is on the average larger, with 4.9 ha, than those using ards, where the figure is 2.2 ha. In 22 % of the regions the arable areas are between 5 and 10 ha and in 9 % between 10 - 20 ha. Only one-fifth of the cropping area of the farms is smaller than 2 ha. Under tropical and subtropical humid conditions, as for example in South Brazil, the implements must often work on fields that have a great deal of organic matter. Many plows show up problems in working in organic material and can easily clog. This particularly applies where the proportion of fallow is high. But also a winter fallow in humid areas or harvest residues can cause considerable delay for seedbed preparation. In trials on seedbed preparation with one implement in South Brazil clogging was found to require 7 h/ha merely for the cleaning of the implement. (Ara1988b)

If on areas heavily infested with weeds or with the breaking of grassland the turf does not smoothly separate and the plow operation is hampered, a coulter can be used to improve the work. Knife coulters can only be employed in heavy soils however; in light soil the plant residues are not properly chopped. They are caught in front of the coulter, lead to clogging, increased draft power requirement and poorer work quality. A disk coulter can perform well with low soil resistance, but it is more expensive. Therefore, the coulter should only be used if the described situation exists. (Preuschen, 1951) Our study showed that the coulter is not used in practice in agriculture, even if it is offered by manufacturers of farm machinery, as is the case in Brazil.In very clayey soil the soil sticks to the plow body and hampers the smooth functioning of the implement (figure E 41). Nevertheless, the high proportion of clay in the soils, as for example in some regions of Paranhas not been conducive to the application of disk implements (as has motor mechanization).

In the regions studied the survey indicated that in practice primarily single-wheel plows are widespread. Gallows plow are seldom used (mainly in Santa Catarina in South Brazil; Viebig, 1988); the same applies for two-share frame plows (Zimbabwe, Botswana). In addition multipurpose implements such as the Policultor and Ariana are available.

Frequently, no exact figures were given by the respondents in the survey whether a conventional plow or a reversible plow was employed. According to Starkey (1989) reversible plows however are only being used in Angola, Madagascar and on some research stations. In the cases which mentioned mouldboard plows in general a classification for the conventional plow was attempted for the African region. The reversible plow is more often found in South America. According to the survey it is commonly used in practice in Brazil. Also, in the Dominican Republic its use on farms has been confirmed. Most of the reversible plows for draft animals in the regions studied are turnwrest plows, which are designed as single-wheel plows. Their working width can only be adjusted if they are equipped with a variable attaching point. Two-way turnover plows are only on offer in combination with multipurpose implements (as the Policultor 600 in Brazil) and find little use in practice; more often they are found in research and extension facilities. (compare Starkey, 1988a) Regarding the weight there is a broad spectrum with these implements. The average of the plows mentioned in the survey is 37 kg. The figures for conventional plows lie between 30 and 70 kg, for reversible plows between 30 and 40 kg. The plows can be manufactured totally from steel or be equipped with a wooden frame.

The weight of the Brazilian reversible plow Tatu no.4 can be reduced from 42 to 36 kg if a wooden frame is installed. Swing plows can achieve a significantly lower weight, for example only 16.5 kg for the Tatu H-5 with a wooden frame. During turning, especially on steep slopes, one loses a great deal of time if heavy plows are used and thus a poor area performance is achieved. In a trial a 72 kg-implement required 105 % more time per hectare than a more efficient plow weighing 42 kg (Tatu no.4 "twin-share" plow type; figure E 42). (Ara1988b; Casa et al., 1988; Figueiredo et al., 1986) A low weight of the implements is particularly important where the plots are distant from each other or are located on a slope and transport to the fields becomes a problem.

The use of the conventional plow is unsuited for slopes. Nevertheless, in many regions in South America and Africa it is employed since it is technically simpler and cheaper (table E 6). Especially where no reversible plow is offered the conventional plow or traditional implements such as the ard are found.

Conventional plows are far more prevalent in flat areas: in 33 out of 48 responses.

Reversible plows only are found in three cases on level to hilly terrain.

The survey showed a slight advantage of the conventional plow over the reversible plow in area performance with 23 h/ha vs. 25 h/ha, respectively. It must be taken into consideration however that the individual figures diverge substantially. In light soils 14 -16 h/ha was mentioned as being usual, whereas in one region also having light soils 42 h/ha were required. In the latter case however these were farms practising a long fallow period (R value = 38) and having serious difficulty in working the soil due to root residues on newly cleared fields. With respect to the reversible plows, primarily turnwrest plows are concerned, whose working width is limited. Their principle advantage in terms of area performance, given by Preuschen (1951) as 10 % for European conditions, cannot be applicable.

Also in trials by IAPAR in Brazil it was determined that the conventional plows examined achieved better results regarding area performance than did turnwrest plows (table E 7). Furthermore, the conventional plows under study had a lower specific resistance with an average 0.38 kp/cm² (best value 0.31 kp/cm²) in clayey soil than the reversible plows with 0.45 kp/cm² (best value 0.34 kp/cm²) and achieved a greater working width (32 cm) than the reversible plows (27 cm). (Ara1988b; Casao et al., 1988; Figueiredo et al., 1986) This may be attributed to the fact that the plow bodies of turnwrest plows cannot be optimally designed.

Reversible plows have only been employed in agricultural practice in some countries to date. Technically and economically viewed however this innovation means a considerable leap ahead. Thus, this implement is found primarily in regions having a developed market structure and a high technical level, as in Brazil. Regarding the turnwrest plow two types are represented: the "pointed share" and the "twin-share" plow type. "Pointed share" plow type

The "pointed share" plow type has a characteristic cylindrical-helicoidal shaped mouldboard (Ara1988b) and a single pointed share. Since the point of gravity lies relatively low the frame can be designed in a curved fashion, whereby the risk of clogging will occur (figure E 43). The mouldboard must be attached correctly to the leg. The device operates well in high weeds and is primarily used on medium soils of the sandy loam type (figure E 44). It is frequently manufactured by artisans, whereby they use pre-fabricated, partially forged plow bodies. Some artisans have difficulty in repairing the implements. Lateral and share pitch change with abrasion. When used incorrectly the plows lose their working characteristics and control stability. Ara1988a) suggests the use of a template for the appropriate reconstruction of the angle.

"Twin-share" plow type

The "twin-share" plow type has a symmetrical plow body with a cylindrical double mouldboard and two separate shares (twin bodies) (figure E 45 and figure E 46). The shape of the plow body requires a higher point of gravity, so that the frame is kept low. In the small space between the share tip and the frame vegetation residues can collect, which can lead to clogging. It is also used in regions having very clayey soils (Oxisols, Alfisols; USST), which behave as sandy soils in a dry condition, but are extremely sticky when moist (see section G 2, case study: Paran Experience with design and maintenance

A large proportion (54 %) of the mouldboard plows are manufactured in national, industrial production. In addition, local fabrication in Brazil plays a significant role. In 21 % of the responses the implements are produced in development projects or imported, which means that the production does not yet have a hold in the country. For imported goods the delivery of spare parts could cause a problem. However, also with a central, national production a well functioning distribution is not guaranteed, as the case of Togo illustrates. It is important to have an infrastructure by way of sufficient available stores and suppliers within reach, and artisans who are able to carry out welding. Design errors were hardly mentioned by the respondents. In most cases the weight of the implements is adapted to the capacity of the animals and the handling ability. Only in Brazil were the implements considered to be too heavy in 25 % of the instances (5 out of 20). Also, the workmanship and quality of the material is hardly criticized, however the purchasing of spare parts was a problem in 33 % of the instances. An exceptional problem of mouldboard plows is the support wheel. Very frequently abrasion of the bearing has been reported. Aside form the constraints mentioned in the general text (section E 2.2), some weak points were also mentioned in the survey. Since shares wear out easily they must be frequently replaced or reworked. With unfavourable soil conditions they are worn out after about 5 ha, equivalent to the work of one season on many farms. Reworking of shares is only done in part according to the survey. When exchanged it may happen that the shares do not exactly fit to the plow body and gaps appear; thus a greater draft power requirement is needed. In some countries such as Malawi and Zambia the quality of the steel is generally criticized.

Finally, it was reported that an insufficient stability occurred on the point of attachment (Zimbabwe, Botswana, Togo). Here bending and even tearing away may occur, often leading to the breakage of the width adjustment. A temporary repair job by local artisans eventually can no longer take place and the adjustment of the working width becomes impossible. The poor adjustment of the plow then leads to an unbalanced load on the handle, which then can permanently become bent.

1 = Retainer of the plow body

7 = Adjustment of the working width

2 = Close connection between plow body and leg

8 = Adjustment of the working depth

3 = Space between plow and leg

9 = Share

4 = Point of attachment

10 = Countersunk bolts

5 = Bearing

11 = Landside

6 = Support wheel

12 = Handle

Fig. E 47: Problematic points on the mouldboard plow (the "pointed share" reversible plow and a conventional plow are used as examples)

Generally, it is a disadvantage that in some countries only one or a few various plow body sizes are available; thus no adaptation to the draft power of the existing animals to the type of soil is possible. In some countries of Africa this multiplicity however could lead to further problems in the procurement of spare parts. In Brazil, on the other hand, for the development of an improved mouldboard plow type for the subtropical humid climate the following demands were placed by IAPAR (Casao et al., 1988):

- various designs for different soil textures,

- different sizes to adapt to existing draft animals,

- conventional plow and reversible plow designs,

- adjustability of the handles to adapt to the different tallness of the farmers,

- suitability for the work in terrain having abundant vegetative residues, i.e. greater space between plow body and frame,

- rapid and practical system for turning the plow body,

- possibility of attaching a disk coulter in order to facilitate the work on fields having thick
vegetative growth.

5.3 Rotary implements: disc plow, disc harrow

Disc implements cut into the soil under the own weight, and not because of the share angle as other soil preparation implements. Thus, they must be very heavy (ca. 200 to 600 kp per disc). When designed to be drawn by animals this implement is equipped with a seat; this idea originated from North America. The disc plow (figure E 48) is not used in the areas investigated in the survey.

The disc harrow (figure E 49) primarily breaks the clods and does not mix the soil appreciably. Therefore, it is useful for levelling and crumbling the soil after plowing and processing residues (working in or scattering); it can be applied as a primary tillage implement on light soils. The working depth is limited to a few centimeters, otherwise the draft-power requirement would be too high. (Wieneke and Friedrich, 1983). Since the discs are arranged on shafts angled to the direction of travel no lateral forces occur with this implement. The angle of the shafts can be adjusted so that the discs till the soil at various cutting angles. Notched discs are better suited for breaking clods and working in organic material. The implements usually are equipped with at least eight discs, have a working width of approximately 1.6 m and a weight of 140 kg.

In the survey the disc harrow was known in 10 cases, all in Brazil. They are offered by several farm machinery manufacturers in south Brazil, but are seldom found in practice. The substantial weight and relatively high purchasing price (five times the reversible plow according to Regencia company in Iratare the main constraints. Transportation of the implement is also difficult. The disc harrow is very common on farms having tractor mechanization. Apparently animal-drawn disc harrows had already been in use prior to tractorization. The implement is not used in Africa (Starkey, 1989), which was also confirmed by the survey.

6. Seeding

6.1 Requirements of seeding

Seeders represent the highest technical demands of all the implements in the survey in the area of animal traction to date. They require precision in manufacturing and assembly, since many parts need to be milled and shaped. As a lack of parts however can hinder the distribution of implements, the technical aspects are dealt with in greater detail here. The experience gained can reveal information on the further developments of draft-animal technology.

In principle, seeding can be done by three methods:

- broadcasting,
- dibbling,
- drilling.

Additionally, there are three procedures for planting or transplanting (tubers, cuttings, seedlings). Broadcasting takes place on the soil surface and is generally done manually. The procedure is primarily applied for small seed that does not require a great seeding depth, as for example wheat and rice. The seed is subsequently worked into the soil by harrowing. After seeding no further work occurs with draft animals.

Drilling procedures necessitate the transition to seeding in rows. This means a higher draft power requirement, so that the use of hand-pushed seeders is only possible on well prepared soils. On the level of animal traction two procedures are applied: manual seeding in furrows, which are prepared by animal-drawn ards (figure E 23), or the use of drilling machines.

Animal-drawn planters, for example for sweet potatoes, potatoes, yams cassava or sugar cane, are not employed in cropping measures in the regions investigated. Star-type planting holers are only known in pilot projects.

A transition from broadcasting to hand seeders or animal-drawn seeders is occurring for the following reasons:

- increase of area performance,
- more exact seed depositing in regard to depth and spacing,
- ease of work by means of greater seeding density,
- maintaining rows more precisely to facilitate subsequent work operations.

A more appropriate adaptation to the various soil fertilities or the planned quantity relationship of adjacent crops in mixed cropping should be achieved by means of the exact maintenance of the seeding density. Further goals are the saving of seed, a more even distribution regarding emergence and maturing and a better distribution in the stand spacing. Saving seed is particularly economical if expensive seed (e.g. hybrid) is being used. An adaptation to the supply of moisture and the requirements of the respective crop can be accomplished with an exact regulation of the depth of depositing the seed. The exact maintenance of the seed depth facilitates an increase of yield for sensitive crops. Sowing in rows is a pre-condition for the use of animal-drawn seeders.

In contrast to simple manual methods the area performance increases fourfold and in comparison to manual implements such as the jab planter it is approximately doubled. According to the survey the area performance of animal-drawn seeders is about 6 h/ha (with maize). In an experiment with Brazilian implements 3.5 to 4.0 h/ha were recorded (Casa et al., 1987). For a manually operated dibbler an area performance of 3.5 h/ha was achieved (Wijewardene and Waidyanatha, 1984), which appears to be very high. Seeding in rows required for animal-drawn implements does not always offer an optimal space for the crops. If the seeds are deposited singly, as is occasionally the case for maize, the seed spacing is less important, for example for precision seeding.

Under certain circumstances draft-animal implements offer the possibility of practising non-tillage on unprepared soil, which can also be carried out with manually operated jab planters or dibblers.

6.2 Implements for seeding

6.2.1 Furrow breaker and row marker

Furrow breakers (figure E 50) are frequently used for keeping orderly rows or facilitating the work with animal-drawn seeders. They are then a particular advantage when organic residues or clods lead to clogging of the implements.

For marking the row spacing simple own designs are appropriate.

6.2.2 Seeders

With regards to seeders one distinguishes between the dibbling and row-seeding method. Dibbling seeders

Pulled or pushed dibbling seeders follow the example developed by IITA (Wijewardene and Waidyanatha, 1984) having a hopper and equipped with a wheel. The tips, which are opened by a lever when the earth is touched, penetrate the soil and release the seed. The lid is subsequently automatically closed by gravity. Because of the fixed spacing of the tips the number of kernels in the row can only be modified by the spacing wheel, leading to pocket drilling. An exact number of plants can however not be assured per row. Dibbling seeders can be equipped with small front wheels for the purposes of transport and maintenance of working depth. The adjustment of depositing depth is done by a press roller.

The implement has the advantage that without possessing any substantial weight it can nevertheless penetrate unprepared soil or cut through a mulch cover. It also hardly becomes clogged in fields having a high proportion of vegetation residues. According to our experience however the implement has in practice proved to be a failure in the survey regions, both as a hand-operated implement and for animal traction. It has a tendency to prematurely trigger the opening mechanism, for example by pebbles or root residues or simply by centrifugal forces, which are too high even at speeds usual for draft horses. Furthermore, the tips become stuck with high humidity or in clayey soils. The seeding mechanism (planting jaws) is subject to breakdown; it closes poorly only if slightly damaged. It is difficult to exchange the seed-plate which regulates the spacing. (Casao et al., 1987; N 1989; v.d Decken, 1989)

In trials with various seeders in Brazil the Grazia rotary injection planter (figure E 52), which did not go into production, achieved the best results in terms of weight, maneuvering time, clogging susceptibility and area performance, and second lowest in draft power requirement. However, it is not suited for planting cotton. (Casao et al., 1987) In order to exploit the basic advantages (no clogging, possibility of seeding in mulch, low power requirement) a further development of this principle would be worthwhile. Its disadvantages could possibly be eliminated by the selection of a different principle for dosage and the punching mechanism (e.g. the spade principle; compare Shaw and Kromer, 1987). Row seeders

For row seeders a distinction is made between drills and precision seeders. According to the survey usually single-row precision seeders are used for animal traction where rainfed cropping is practised. The seeders have been partially developed for the regional prevalent cashcrop, e.g. the Super Eco for sowing groundnuts in Senegal (figure E 53). Usually, the adaptation of designs to other crops took place later. A compromise had to be found between the precision of seeding of individual crops and the suitability for various crops, since the procurement of special implements is not worthwhile otherwise. Multi-row drills for small-kernel seed are not used in practice in the area surveyed. Various models of direct drilling machines do exist in Brazil for the purposes of experimentation.

These seeders are suited for the planting of larger seed such as maize, beans, groundnut and soybeans. With alteration some implements (e.g. most Brazilian seeders) can plant undelinted cottonseed. In Senegal the Tamba implement has been developed for this purpose; it is used for pocket drilling. Precision seeders are less suited for planting small-kernel seed, which can more easily be distributed by slide and cam wheels than by holediscs or spacing wheels. Nevertheless, precision seeders are often used for sowing of millet and sorghum.

To date there are no special seeders for ridged crops which allow the track, the frame height as well as the position of the press roller to be adjusted for width and height of the ridges. Furthermore, the seeders are notorious for their poor stability on the ridges. For this purpose Nolle (1984, in: Bordet et al., 1988) states that a drawbeam is required. He suggests a prototype having a stabilizing furrow opener, adapted to the ridge shape and the design of ridger. Another problem is that the farmer cannot walk on the ridges and that in any case two draft animals are required. The combination of mechanical ridging and seeding is not satisfactorily possible due to these special requirements.

The poor adaptation of seeders to sowing on ridges is also due to the fact that in some countries, e.g. Senegal, ridging is not the object of a development programme and under certain circumstances seeding is not a bottleneck in regions having ridged crops due to the longer vegetation period. (Havard, 1988a; Bordet et al., 1988)

The precision seeder requires the farmer to adjust the implement to the respective seed by exchanging the hole disk. Generally, precision seeders demand clean and calibrated seed. If no hole disks are available for the desired crop the farmer can not utilize the implement, if he does not want to risk considerable damage to the seed or too high seeding density.

Marking discs are seldom used on seeders. This is possibly attributed to the fact that precise seeding in rows is first required when multi-row weed control becomes necessary. Experience with design and maintenance

Drive and distribution mechanism

Most Brazilian seeders have a front wheel which operates the drive e.g. the Sans seeder The press roller propels the drive of the Brazilian Triton (figure E 55) or the Safim in southern Africa. The transfer mechanism can be simplified by means of the side wheels in the Super Eco (figure E 53). Under certain circumstances utilization is also possible on low ridged crops (Metzger, 1988; v.d. Decken, 1989). Implements with two wheels are more susceptible to clogging and can hardly be used on slopes. The Super Eco is only suited for light, well prepared soils. It tends to plug up where weeds, vegetation residues and moist soil exist. The drive wheels must turn freely and faultlessly in order to assure an even seeding density. If they are not equipped with a tread (e.g. Super Eco) they can slip on loose soil. Also, larger clods or failing to grease the drive wheels can lead to a blockage of the drive. On slopes the front wheel tends to deviate from the row.

A special gear must be placed between the drive wheel and the dispensor in order to be able to deposit the desired amount of seed. Generally, the gear cannot be adjusted, but rather the seeding density is adapted to the selected spacing wheel (metering mechanism). Moreover, the transfer path should be as short as possible to prevent the chain from slipping off (e.g. when turning). Also it should be avoided designing machines with too many bearings and cog wheels. Finally, it should also be possible to disengage the fertilizer and seed distributor by means of a clutch when the end of the row is reached.

Most seeders have either a horizontally or diagonally mounted dispensor. The sloped attachment of the hole disk, as with the Super Eco, is advantageous for sensitive seed such as groundnut (Wieneke and Friedrich, 1983). The dropping distance of the seed should be kept to a minimum, in order to achieve a most possibly exact depositing of the seed. The planting wheel can be designed as a hole disk, seed plate or spoon-fed mechanism. The latter are less sensitive towards calibration errors. For small- kernel seed (e.g. sorghum and rice) only pocket drilling can be achieved with precision seeders.

On the whole, the inadequate precision of the distributor mechanism is criticized. The poor design of the dispensor frequently causes damage to the seed. In the survey fault is found with the manufacturing quality of the hole disk and the conveyor wheel (cast iron), and polishing is suggested as a solution. A figure from Brazil states that 5 - 10 % of the seed is damaged by the spacing wheel. This is confirmed by Casao at al. (1987), who record a 5 % damage rate with maize. Thus, the spacing wheel as well as the seed knockout and dispensor should be as smooth as possible (made of plastic), in order to avoid damage to seed. The holes of the planting wheel should be slanted from below to prevent blockage of the seed. Erroneous mounting can be hindered by means of the recessed design.

Faults have been described for both Brazilian seeders and the Super Eco (Casao et al., 1987; Starkey, 1981). This also certainly occurs with artisanally manufactured planting wheels. The seed of smallholders is often not calibrated. Frequently, the appropriate planting wheels are lacking for certain crops, and their thickness is sometimes not uniform. Many farmers have severe difficulty adjusting the implements.

The hole disk of Brazilian seeders must be replaced by a cogged wheel necessary for transporting to plant cotton. The expulsion takes place sideways by means of a fluted roller. For groundnuts a covering lid is inserted.

The Tamba developed in Senegal for the sowing of non-defibered cotton seed has a distribution mechanism consisting of a stirring apparatus in a housing and a fluted roller under the seed container, which regulates the expulsion. It deposits the seed in pockets, but has been poorly assessed because of fluctuations of seed density (Havard, 1988a). Seeders are delivered equipped with various planting wheels. The Brazilian made HMC (figure E 57) is normally offered with 5 hole disks having 4, 5, 6, and 10 holes, a disk without holes, which can be fitted by the owner, and the dispensor for cotton. The apparatus for groundnut seeding must be ordered as an accessory.

Opening furrows, depositing and covering of the seed

The furrow should be pointed at the bottom in order to prevent rolling of the kernel in the furrow. Sabre-type shares sometimes disk shares, are used to open the furrows. In hard soils hoeing shares tend to glide less than gently curved sabre-type shares (Havard, 1988a). Disk shares are better suited for fields having roots, over which they can glide. However, they can become damaged by stones. As applies for all turning parts, they are expensive. The Triton seeder from South Brazil is the most reasonably priced implement on offer, according to information provided by the farm machinery outlet Regia; they are equipped with double disk shares and have eliminated the front wheel (figure E 55). The Super Eco from Senegal has a knife coulter to facilitate the penetration of the furrow opener. It was however not accepted by the farmers (Havard, 1988a).

With the occurrence of larger clods or vegetation residues the furrow opener can become clogged. Wide shares heap up a considerable amount of earth and organic mass. To improve the work prior furrow breaking is recommended. In some cases clogging can be a result of farmers wanting to sow immediately following a rain as is done in manual operations (v.d. Decken, 1989).

Substantial fluctuations in the precision of depositing the seed have been observed with the seeders. In a trial with Brazilian seeders, average deviations of 40 % were determined in the rows, approximating that of dibbling. The depositing depth is generally adjusted with the aid of the press roller.

However, on some implements the depth adjustment is only possible by means of the point of attachment (e.g. Safim, southern Africa). Aside from the dibbling seeder (Grazia, figure E 52) most implements can maintain a maximum depositing depth of 4 cm (Casao et al., 1987). The Super Eco (figure E 53) is also considered inadequate, since it is not suited for sowing seed deeper, e.g. millet at 6-8 cm on dryland. The regulation of the depositing depth is accomplished by adjusting the furrow opener.

Frequently the farmers get the furrow opener welded, so that it does not become lost (Havard, 1988a). Generally, reports are often heard that serious problems occur with the depositing depth of the seed. Seed covering scrapers and press rollers take care of covering the furrow with earth and assure good soil resealing. Thereby, the moisture supply is sufficiently guaranteed. The seed covering scrapers must cover the furrow properly with soil without allowing organic material to be drawn in or stones to be transported into the furrow which depends on the mounting position. They should be easy to adjust for height and angle (as with the Sans models, figure E 54) in order to adapt to the field conditions at all times and to prevent clogging. The Super Eco is equipped with seed covering scrapers in the form of duckfoot shares, which simultaneously achieve weed control. Faulty function of seed covering scrapers is often mentioned in regard to clogging and poor covering of the soil, especially if they are attached too close to the ground and the press roller.

The shape of the press roller is important: it should provide for optimal covering and resealing by means of the shape of the wheels; larger wheels are preferred in order to avoid clogging. Generally, the press rollers are only adjustable for regulation of the depositing depth.

Application of chemical fertilizer

All implements, including the Super Eco and parallel developments in neighbouring countries, are offered with attachments to spread chemical fertilizers (2/3 of all the cases). Thereby a more precise spreading in comparison to hand spreading is achieved, a saving of fertilizer and more rapid access by the plants. A further aspect is easing the workload. The farmers however must be able to precisely adjust the dosage.

Seeders that can simultaneously spread fertilizer are generally longer and heavier, whereby the machine then becomes clumsier to handle. Fertilizer application occurs through an auger or stirrer and dispensor. Separate depositing of seed and fertilizer is important, so that the seed does not "burn", as has been reported form Zambia.

There are essentially three solutions for separation. The best separation is achieved if the fertilizer is deposited with a detached share from the side (Baldan, Safim) or is placed under the seed. In the latter case usually a second deeper share precedes the first following the same line (Sans). On a dibbler a second, complete planting wheel with a beak tip is attached, which takes care of a clean separation (Grazia). On implements with two shares, especially if there is a staggered arrangement, more draft power is required and risk of clogging increases.

On some implements (e.g. HMC, Triton, Tatu, figure E 58) only one share is available for sowing the seed. The fertilizer falls in front of the seed onto the ground and it is subsequently worked in with the share. This is considered to be an adequate solution (Casao et al., 1987).

In many countries the dissemination of chemical fertilizers is a component part of the extension services. If they are applied however, in practice it is often found that seed and fertilizer are deposited in separate work operations. This can be attributed to common cropping practices as well as to the weight of the seeder.

Material and design

High weight (e.g. Tatu) as well as a high point of gravity (Baldan) make the handling of the implements awkward, especially on slopes and in small plots. Seeders with a light weight can easily be lifted when they become clogged. There is however a minimum weight originating from the seed and fertilizer, for example the seed and fertilizer hoppers of Tatu implements weigh 13 and 23 kg, respectively. The average weight of the implements occurring in the survey was 43 kg. The seven implements tested in Brazil weighed between 53 and 72 kg. If the implement is too long it is difficult to handle on hilly terrain having curves (HMC). Single-row implements have a width of 40 to 58 cm (Tatu). Two-wheeled implements are wider, e.g. Citra = 96 cm (figure E 59).

All bearings should have greasing points to prevent sliding of heavy-geared drive wheels and thus a disruption of the seeding operation (Casao et al., 1987). The hoppers should be easily removeable. This is necessary for emptying, adjusting (exchange of spacing wheel) and cleaning operations. Especially implements which spread fertilizer should be cleaned regularly to prevent corrosion, which is a frequently occurring problem. This could be solved by manufacturing the part out of other material (e.g. fiberglass), but it would entail a higher cost and the new material would create additional difficulties for the artisans.

In the survey numerous problems were listed regarding unsatisfactory design, negligent fabrication and maintenance. The material is predominantly considered to be of medium to poor quality and the high weight is criticized. The bearings cause further problems. Poor assembly causes cumbersome access to bearings, missing sealings lead to the breakdown of bearings, especially due to sand.

In some cases the bearings can only be replaced by the manufacturer. The risk of down-time is high: breakage of chains, gear parts and bolts, wear to gear wheels in the drive mechanism, loose bolts and loss of individual parts were mentioned in the survey.

The required draft power is dependent on the weight and the parts which come into contact with the soil. The Super Eco requires 20 kp on sandy soil and 30 kp on more clayey soils (with coulter, furrow opener and duckfoot shares) (Havard, 1988a). In a test with Brazilian seeders measurements of 20 to 30 kp were also recorded. The Grazia dibbler required the second-lowest draft power after the HMC implement, which only has one share for seed distribution. (Casao et al., 1987)

From a technical point of view an improvement of seeders, the design of which has not been further developed for many years, could be achieved by a lighter construction with modern materials. At the same time, demands on seedbed preparation could be reduced. As a result a direct drilling machine could be created that would work more independent of the field conditions. It appears that such an implement however could not be easily marketed in sufficient quantities, given today's problems which the farmers have regarding the mechanization of the seeding operation, especially due to the high investment cost. Discussion

On the whole, seeders are seldom utilized, only in 30 cases in our survey, including the cases of very occasional use. Limiting conditions for their employment are elucidated in the following assessment (in brackets the number of instances):

- Economic reasons rank first, such as too high price (17) and too low labour and productivity distribution margins (14) (available labour force, labour savings too small or cropping area not sufficient).

- Limited conditions for utilization due to topography (8), unsuited soils (e.g. too clayey and sticky in Zambia and Ethiopia) (4), mixed cropping or lack of row cropping (6), obstacles on the fields (6) and poor seedbed preparation (3) rank second.

- Third follows the lack of adaptation to the agricultural farm system, including the fact that the implement is not known or not obtainable or it does not have any tradition of use or that animal traction is still in the introductory phase (9).

- Finally, it was mentioned that the use of seeders is difficult for the farmers (adjustment, handling), little know-how is available or more extension services are necessary (5). In fields with roots or stones and much organic matter (fallow and harvest residues) seeders do not function properly. The seedbed must be prepared well for most implements, if the soil is not very light. Calibrated seed is necessary for the precision seeder, which hardly applies for the smallholders. Patchy and double sowing can be caused thereby. In addition, inadequate quality of the implements in terms of the work result is often recorded. In comparison to other implements there is a higher number of worn out parts and points to repair; this requires greater preparedness of the farmers to carry out maintenance and more experience by the artisans.

Multi-row seeders have hardly found acceptance in the praxis. They require a greater investment. A better seedbed preparation is necessary and their handling is rendered more difficult due to the great width and weight. An entirely exact seeding in rows is only required, on the other hand, where multi-row weed control is being carried out.

In the regions examined, seeders are primarily being utilized under the following conditions:

- In regions having a short vegetation period such as Mali and Senegal seeding represents a work peak and rapid sowing is necessary, which can in part take place by direct sowing. This also allows an expansion of the cropping area.

- Their acceptance is particularly high for the cropping of cotton and groundnuts, for which they were partially developed. If the investment does not pay in terms of a better yield, the farmers cannot pay the high price. The market production must therefore be quite advanced.

- The supply of accessories (e.g. hole disks) and spare parts must be guaranteed by an industrial and artisanal structure.

Animal-drawn seeders have been, due to their limitations, only disseminated in a few countries such as Brazil, Senegal, Mali and southern Africa.

6.2.3 Fertilizer applicators

In some countries such as Brazil fertilizer applicators are increasingly being used, independent of the sowing time. They are appropriate for the spreading of chemical fertilizer, dry and organic manure. In part this is combined with furrow breaking prior to seeding. Some implements have two applicator tubes in order, for example, to apply nitrogen directly to the plants when passing between the rows. The applicator mechanism consists of augers or stirring devices with spreaders.

7. Weed control

7.1 Requirements of weed control

Depending upon the climatic zone, weed control is one of the most labour-intensive operations, especially if it is carried out with the hand hoe.

The importance of weed control also depends on the competitive forces of the respective crop as well as the amount of time required to build a canopy. The more rapidly the plants grow, the more effective is the shading of the soil as a weed control measure. According to rank, crops having a small spacing possess a relatively greater competitive force. Maize and rice are very sensitive crops due to their slow initial development.

For permanent cropping and high weed invasion it is necessary to apply weed control measures already prior to seeding, since with mechanical hoeing devices only an average success rate of 50 % can be achieved (Walter, 1990). Moreover, the weed-control effect of the implements is poor in row crops. An intensive soil preparation and an additional work operation, for example with the harrow, directly before seeding shifts the competitive conditions in favour of the crops. Seeding must take place in parallel rows, as the cultivator works at a constant width. A very high and more lasting effect can be accomplished if the weeds are eliminated during the first germination phase. Simultaneously, a better aeration of the soil is achieved, the infiltration rate of the water is increased and the surface capillaries are destroyed; thereby the evaporation of moisture is hampered. As long as no root system has already developed, weed control should begin as early as possible -when the first weeds appear - in order to eradicate the germinating weeds. It should be done as close to the surface as possible, a maximum of 3 - 5 cm deep, in order not to raise lower lying weed seeds to promote their germination. For the farmer however the field is still clean at this stage of development and their is no reason to intervene. (Almeida et al., 1983)

The number of work operations fluctuates depending upon the crops and the growth of the weeds. The later the weeding takes place, the greater the root penetration of the weeds, demanding deeper working of the fields. The subsequent work operations depend on the type of crop, the climate and weeds; here superficial work operations are recommended in order to prevent damage to the root system of the crops.

The practice of mechanical weed control is receding with the increasing application of herbicides, leading directly to problems of toxicity and cost. Contact of the poison with the skin, including the sensitive parts such as the eyes and mouth are unavoidable. Masks and protective clothing are seldom worn (figure E 60 Use of herbicides with draft-animal implements: The boy drinks a coke after the work, in his own words "to neutralize the effect of the poison.")

The problems connected with chemical weed control have given rise to an increasing interest in green manure in Brazil; its use can control weeds when done at the correct time.

Chemical weed control can save a great deal of time. It is often employed where labour forces are scarce and its use makes weed control on stony fields much easier. However, it has been reported from Brazil that results are not satisfactory because of underdosages and the subsequent appearance of resistance of the plants. In mixed crops the problem occurs that herbicides are usually only suited for one of the two crops (Vieira, 1985).

7.2 Implements

One-, three-and five-share cultivators, ridgers and ards are employed as implements for weed control. They have already been described under the section on soil-preparation implements, since they are often identical with them.

The advantage of the single-share hoeing (figure E 61) implement is its multipurpose use and simple handling. It can be employed where obstacles occur and is effective where high weed growth is present. Also it can be applied for most crops and with mixed cropping due to the narrow working width. Since a width adjustment and wheel are not incorporated into the machine for depth adjustment, there is a severe risk that roots may become damaged; this is frequently criticized by the extension services. The implement is sturdy, easy to manufacture and to repair, and has a reasonable cost. It is offered by local artisans and farm machinery distributors.

Multiple-share cultivators (e.g. Houe Occidentale, Houe Triangle, Houe Manga, figure F 16, F 7) can be adjusted in the width and various small tools (e.g. bar-point share, duckfoot) can be mounted. The working depth can be regulated by the support wheel. Thereby the danger can be reduced that the roots of the crops will become damaged. When the cropped plants are still small it is recommended to carry out weed control with narrow tines, since they only move the earth slightly to the side. Larger plants are less sensitive to this problem. In this case wider tools (duckfoot, sweepshare) can be used for a shallow working of the soil; this prevents damage to the roots.

The Planet cultivator (figure E 62) was developed at the turn of the century by the Frenchman Planet as an "ideal" cultivator. In Brazil it is sold by Sans, Tatu and Baldan companies. Its design allows manifold possibilities of adjustment. Nevertheless, it is little used, since the price for the farmers is too high. In addition, it tends to clog.

Usually three-share cultivators are used. Five-share implements are offered by the distributors but are seldom found in practice. In Brazil three-share cultivators, sometimes with adjustments, are also fabricated by local artisans (figure E 63). The multi-share cultivators are usually of the adjustable type. From the survey it was established that in Brazil only part of the artisanal-manufactured implements and in Botswana only one cultivator type mentioned in the survey was not adjustable. The width adjustment can be done by loosening the fixture and resetting the tines, as is the case with the cultivators in Senegal, or by modifying the frame width by a lever or spindle, as is done with the Planet cultivator, or by loosening a clamp bolt.

Ridging has an effective result for controlling weeds in rows. Sufficient earth is heaped up against the plant which simultaneously covers and smoothers the weeds. No other implement is available which so successfully achieves this purpose in rows. However, heaping up is only possible with larger plants; smaller crops could become damaged.

7.3 Distribution and Experience

Weed control with animal-drawn implements is done in 73 % of the cases (51 out of 70 instances) in the regions investigated. In Brazil in 87 % of the cases (20 out of 23 instances) cultivators are used in agricultural practice, while in the African countries they are common only in 64 % (25 out of 39 instances) of the cases.

In African countries ridgers and multi-share hoeing implements were mentioned for weed control. In practice however these are only utilized under certain circumstances. The existence of a work peak and the cropping in rows tend to favour their use. Cropping without any particular order, broadcasting and mixed cropping tend to be a hindrance for their application. The application of the ridger for weed control purposes is associated with the regions where ridge cropping dominates. This is the case in some regions in Togo, Ghana, Cameroon, Malawi and Zambia. Cultivators are mainly used in Mali, Niger, Burkino Faso and Senegal, where shallow seedbed preparation is common.

In Brazil three-share cultivators having single-sided ridging shares are used (figure E 63). Single-share cultivators with broad swallowtail shares or a hovel-shaped tool, e.g. bico de pato (figure G 26) or the small fuor (figure E 61), are utilized. On the other hand, ridgers are seldom employed for weed control.

Permanent cropping is almost always found in regions where mainly multi-share cultivators are used. Correspondingly few obstacles exist and the occurrence of stones is minimal. In the Andes countries and Ethiopia the ard is employed for weed control. In Ethiopia this work operation is seldom conducted since teff, the primary crop, is broadcasted.

Generally, difficulties were mentioned regarding the training condition of the animals and the too late weed control. Weeds are frequently not removed before they reach a height of 20 - 30 cm. If weeding is done by hand the weeds can more easily be gripped and can be completely pulled out. However this development stage of weeds has gone beyond the bounds of effectivity for the mobilization of implements.