| Agro-forestry in the African Humid Tropics (1982) |
|Traditional agro-forestry systems: Prospects for development|
Crop mixtures in traditional systems
Akinola A. Agboola
Professor of Soil Fertility and Farming Systems, Department of Agronomy, University of Ibadan, Ibadan, Nigeria
The traditional cropping system is stable because it is adapted to the farmers' level of technology and the soils' capability. It incorporates mixed cropping and bush fallow, and it gives a high total return per unit area of land. Furthermore, growing crops in mixtures is consistent with the farmers' goal of security. Their present systems have evolved naturally as an answer to the challenging environment in which they live.
Researchers have been hesitant to tackle multiple cropping experiments in general, and agro-forestry in particular, because of the infinite combinations possible, lack of knowledge about existing systems, and the traditional separation between agriculture and forestry. Also, multiple cropping is associated with unmechanized farming and low productivity; research in intercropping and multiple cropping should be geared to increasing the productivity and returns in both arable crops and forest products. The peasant farmers' system of agro-forestry should be improved upon, and researchers should evolve a combination of arable crops and fast-growing trees that can be easily adapted by smallholder farmers.
The traditional cropping systems will continue until an alternative is evolved that can fit into present technology, environmental constraints, and at the same time maintain high crop yield. My feeling is that agro-forestry research has the potential of offering an early and viable alternative.
Farming systems can be defined as the distribution of plants and animals in space and time and the combination of inputs believed to give maximum production in socioeconomic, political, and cultural contexts.
In Africa, a farmer or farm family usually operates a small, diversified agricultural enterprise. According to Okigbo (1978), farmers with a homestead in an upland, well-drained soil may operate a compound farm or garden close to their homestead and maintain two or more plots in cropping systems involving natural or planted fallow and in the flood plain of a nearby river or steam. They may also keep pigs, goats, sheep, and poultry for manure, meat, sales, etc.; they may be palm wine tappers, basket-makers, musicians, or priests in the traditional religion. The cropping mixtures on the farms often involve major staples, vegetables, and condiments in multiple, double, relay, and patch intercropping patterns of annuals, perennials, or both. The compound farm or homestead garden usually carries more species of cultivated plants than bush fallow farms.
The most common tradition in African cropping systems is the spatial arrangement of crops on the field. The crops are established haphazardly in mixed culture (Okigbo and Greenland 1975), the objectives being to take advantage of local topographic features and micro-relief; disperse species at wide enough spacings so that they do not compete for nutrients and light; ensure that crop cover is adequate to control soil erosion and weeds; and ensure that each species's requirements for sunlight are met. Where annual staples are uniformly planted among tree crops, heavy pruning of the tree crops is usually carried out to ensure that adequate light reaches the ground level. Whether crops are grown on mounds, beds, ridges, or the flat, their spatial arrangement and frequency in mixtures usually indicate their importance in the diet and sometimes their uses.
Intercropping and Multiple Cropping
The simultaneous cultivation of different crops on the same piece of land has been described interchangeably as mixed cropping or intercropping by Webster and Wilson (1966) and Norman (1971). Ruthenberg (1976), however, distinguishes between mixed cropping and intercropping on the basis of the pattern of the intermixture.
The term intercropping has been used rather generally in the literature as referring to the practice of growing two or more crops simultaneously in different but proximate stands (Okigbo 1978). Grimes (1963) defined a common practice in intercropping: the system of growing different crops in alternate rows, which he terms alternate row cropping. Row intercropping is common in filled areas, annuals often being planted under perennials. For instance, tall-growing crops such as cassava or bananas are planted in young coffee, cocoa, or rubber plantations (Sanchez 1979).
In multiple cropping, Herrera and Harwood (1973) indicated that each of the crop mixture patterns has different physiological characteristics and different advantages. For example, Norman (1974) showed that although there were at least 156 crop mixtures and many different spatial arrangements among Hausa farmers near Zaria (Nigeria), the most popular arrangement was a systematic spatial pattern on ridges. With intensification of cropping, interactions among plants become critical. The most widespread multiple cropping systems practiced in the humid tropics are mixed intercropping and relay intercropping.
Mixed intercropping is common when cereals, grain legumes, and root crops are grown together and when little or no tillage is practiced. For example, farmers in southern Nigeria plant simultaneously maize, cassava, vegetables, and cocoyam. In Abakaliki, Nigeria, mixed cropping is practiced in mounds or ridges of soil constructed with hoes. Several crops are planted on different parts of the mounds. For example, an Abakaliki farmer plants yams on the mound, rice in the furrow, and maize, okra, melon, and cassava on the lower parts of the mound. Mounding is beneficial because it increases the volume of soil available to root crops.
Relay intercropping is a practice where a second crop is planted after the first crop has entered the reproductive growth phase but prior to harvest. A common example is the maize-beans system used in most of Central America and much of tropical South America. Maize is planted in rows, usually at the beginning of the rainy season; when the ears are well formed, farmers break the stalks just below the ear and plant climbing bean varieties. Relay intercropping is also very common in rice-based systems in Taiwan. Up to five crops per year can be harvested by two relay successions, rice-melons followed by rice again relayed with cabbage and maize. At present, the maize-cassava relay is being developed at the University of Ibadan, with researchers studying the effects on soil nutrients.
Advantages and Disadvantages
Baker and Yusuf (1976) wrote that the almost universal practice of traditional cropping systems by subsistence farmers throughout the world is an indication that the system has evolved naturally as an answer to the challenging environment.
The rationales for crop mixtures are that they may be relatively more profitable than sole cropping (Chandra 1978), the difference between the marginal value product of resources and the opportunity cost of the resources being insignificant (Norman 1974); they are consistent with the goals of security and year-round subsistence needs (Andrew 1972); they may alleviate adverse conditions in the ecosystem; and they may maximize the space, water, and nutrients available. Some of these benefits can be further promoted by good tillage practices, based on the principle of minimizing disturbance of the ground and vegetative cover. The practice of minimum tillage, with ample crop residues left on the soil surface, has great potential.
Although monocropping tends to attract fewer diseases and insects, these are more likely to be highly prevalent and to cause considerable damage. Cropping mixtures may reduce the abilities of pests and diseases to spread. For instance, interplanting has been shown to reduce insect problems in groundnut-sorghum and cassava-maize mixtures and has reduced the incidence of bacterial blight.
The denser plant population usually found in crop mixtures may also help control weeds (FAO 1968). In addition, because crops mature at different times, mixtures may extend the period of the year during which the soil is protected by leaf cover and root systems (Igbozurike 1971).
The disadvantages of traditional systems are that there is reduced yield of the component crops (Chandra 1978; Webster and Wilson 1966; Agboola and Fayemi 1972); there may be competition for light, nutrients, and water (Dalal 1974; Willey 1979; Webster and Wilson 1966); there may be allelopathic effects due to excretion of toxic substances by one or more crops (Dalal 1974); the practice is not well suited to modern agriculture or mechanization and, thus, research on traditional systems has been inadequate (Ahmed and Gunasena 1979); and suitable methods for investigation are difficult to define (Haizel 1974).
Unfortunately, research workers appear to have been hesitant to tackle multiple cropping experiments because of the many crop combinations in use, and because multiple cropping by the peasant farmer is associated with non-mechanical farming and low levels of productivity. Doubts have been expressed as to whether any of the positive benefits of multiple cropping can be exploited at more advanced levels of farming. Attempts to improve production by the application of technology developed in temperate cropping systems have failed in Nigeria and in most other tropical countries, not because of farmers' conservatism but because the approach is inappropriate.
Farmers in the tropics have been noted to grow complex crop mixtures on compound farms, especially in the rainforest zones, where staples, vegetables, and perennial fruit trees are interplanted. As early as 45 years ago, Leakey (1934) observed that the relay and mixed cropping practices had many obvious advantages, and he recommended that those involved in agricultural development should give serious attention to research on traditional food production systems, especially those involving intercropping. This recommendation has largely been ignored, despite the fact that peasant farmers have repeatedly refused to adopt monocropping practices recommended by extension agents.
The main objectives of research into the productivity of mixtures might be to screen mixtures for high-yielding combinations; to test alleged advantages of traditionally grown mixtures; and to gain an understanding of the processes that lead to advantages so that, in a specific environment, a rational choice of components may lead to higher yields than are possible in monocultures.
Sturdy (1939) noted in East Africa that intercropping Crotalaria with millet, and groundnuts with sorghum, helped in the maintenance of soil fertility. Lambers (1940) reported that coffee intercropped with bananas in Kuri provided a mulch that improved the fertility of the soil. Results of experiments carried out by Agboola and Fayemi (1972) showed that legumes intercropped with early maize gave a maize yield equal to that obtainable with 55 kg/ha of nitrogen supplied as mineral fertilizer.
Much work still has to be done to quantify the nutrient level of soils under traditional crop combinations. In a study conducted in 1981 composite soil samples were taken from eight farms and their adjoining fallows, and in only one case was the fallow land significantly higher in nutrient status than the adjacent cultivated land. P, K, Fe, Zn, and Ca levels in the top 0-15 cm layer were higher in the cultivated fields than in the adjoining fallows, and this was attributed to the effect of burning after clearing. These data support the view that the length of the fallow has been drastically reduced, thereby reducing also the nutrient build-up. They imply that most farmers are actually cropping infertile lands and that the nutrient status of the field does not influence the farmers" choice of crop combinations. The choice of crop combinations seems instead to be influenced by the food staples usually planted in the area, and the inclusion of vegetables is related to the economic value of the crops and food preferences. No definite effect of the crop mixtures on the soil nutrient status could be established.
Summary and Conclusions
The best cropping system in the tropics, once the soil is considered as the main factor sustaining crop production, is one that will not expose the soil to erosion hazards Therefore, more research is needed to identify better combinations, including agro-forestry schemes, so that the best type of rotation can be developed for each ecological zone. Although zero tillage has been advocated by IITA, this cannot be practiced ail over the humid tropics because it is dependent on the soil, particularly the clay content, and the prevailing weather conditions. In some areas, the temperature is low during the harmattan, and the Fadama soils cake, necessitating extra soil preparation in the following rainy season. Thus at the University of If e zero tillage has proved to be unsatisfactory.
Besides protecting against erosion, an appropriate cropping system must guard against the breakdown of the soil structure and nutrients. A good system needs to take advantage of the tonnes of human, plant, and animal refuse being produced. It should be emphasized, however, that traditional farmers will continue with their existing system until an alternative is found that will maintain higher yields, conserve the soil, control weeds, and generally fit into their present technology.
The author appreciates the contributions from J.M.A. Tocunana and C.F. Yamoah, both postgraduate students of the University of Ibadan's Department of Agronomy.