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Sustainable agriculture
Farming systems of tropical Africa and their sustainability under changing conditions
Ingredients of sustainable farming systems and issues to be considered in the design of these systems
Sectorial interface requirements
Conclusions and recommendations
References

Bede N. Okigbo

The World Commission on Environment and Development (WCED 1987) defined sustainable development as development that meets the needs of the present without compromising the ability of future generations to meet their own needs. Although I do not feel like adding to the unending list of definitions of sustainable development, there is need to consider definitions that make it easier to better conceptualize the nature and processes involved in sustainable development rather than the usual listing of characteristics of the term(s). Here, I attempt to present an operationally practical definition of sustainable development as the current global development paradigm consisting of policies, plans, programmes and activities of conserving, managing and utilizing resources to at least satisfy basic needs and improve human welfare by employing such strategies, technologies, processes and systems of production that do not degrade the resource base, cause losses or changes in the environment that are ecologically, economically and culturally undesirable. According to the WCED (1987) definition, the changes that are ecologically, economically and culturally undesirable are those that damage the environment to the extent that future generations will find it more difficult to find ways or generate technologies for rehabilitating the degraded environment and/or reversing the adverse changes in order to utilize environmental resources to meet their needs and ensure human welfare than we are finding it easy today to achieve the same objective. In other words, sustainable development consists of practices and techniques of managing and utilizing resources to fulfil human needs without damaging the environment so badly as to make it more difficult for future generations to manage and utilize environmental resources to satisfy their own needs.

Sustainable agriculture

Sustainable agriculture has been variously defined as follows:

1.  

• the successful management of resources for agriculture to satisfy changing human needs, while maintaining or enhancing the natural resource base and avoiding environmental degradation;
• the ability of an agricultural system to maintain production over time in the face of social and economic pressures;
• one that should conserve and protect natural resources and allow for longterm economic growth by managing all exploited resources for sustainable yield (BIFAD 1988).

According to Dover and Talbot (1987), although sustainability means different things to different people, on the basis of ecological principles sus tainable agricultural systems are those whose productivity can continue indefinitely without undue degradation of other ecosystems.

2. According to SCA (1991), sustainable agriculture is the use of farming practices and systems which maintain or enhance:

• the economic viability of agricultural production;
• the natural resource base; and
• other ecosystems which are influenced by agricultural activities.

3. A system synonymous with alternative agriculture, which is any system of food or fibre production that systematically pursues the following goals:

• more thorough incorporation of natural processes such as nutrient cycles, nitrogen fixation, and pest-predator relationships into the agricultural production process;
• reduction in the use of off-farm inputs with the greatest potential to harm the environment or the health of farmers and consumers;
• greater productive use of the biological and genetic potential of plant and animal species;
• improvement of the match between cropping patterns and the productive potential and physical limitations of agricultural lands to ensure long-term sustainability of current production levels; and
• profitable and efficient production with emphasis on improved farm management and conservation of soil, water, energy and biological resources (BOA/NRC 1989).

These agricultural systems employ a broad spectrum of practices that include:

• crop rotations that mitigate weed, disease, insect and other pest problems; increase available soil nitrogen and reduce the need for purchased fertilizers; and, in conjunction with conservation tillage practices, reduce soil erosion;
• integrated pest management (IPM), which reduces the need for pesticides by crop rotations, scouting, weather monitoring, use of resistant cultivars, timing of planting and biological pest controls;
• management systems to control weeds and improve plant health and the abilities of crops to resist insect pests and diseases;
• soil- and water-conservation tillage;
• animal production systems that emphasize disease prevention through health maintenance, thereby reducing the need for antibiotics;
• genetic improvement of crops to resist insect pests and diseases and to use nutrients more effectively (BOA/NRC 1989).

In this paper, for a discussion on criteria for designing sustainable agricultural systems, I hope you will bear with me for introducing yet another definition. Sustainable agriculture is the science, art and business enterprise in which the farmer manipulates environmental resources and orchestrates several inputs in amounts, quality, sequences and timing in order to bring about or "create" environmental conditions that favour the production of plant and/ or animal products needed for food, fibre and other products without causing environmental degradation and decline in yields. Sustainable agriculture in any given location can only be achieved where there is appropriate scope of research linked with extension and the farmer to ensure that there is knowledge, skill, understanding and technology for manipulating and dealing with:

• the physico-chemical factors, such as soils, climate, moisture, radiation, day length, etc., and the way they change and interact so that they can be manipulated or given due consideration in efforts aimed at creating favourable conditions for
• the biological elements of the production system in terms of crops and/or animals whose products are required in relation to their interaction in the agroecosystem with weeds, pests, and even beneficial and non-beneficial organisms that shape their environment on the basis of
• changing and appropriate technologies at the disposal of the farmer, and acceptable and relevant to his circumstances on the basis of his
• sociocultural background, in relation to education, experience, community organization, social relations and institutions, legal systems, etc., to the extent that they interact compatibly to determine
• the economic viability and ecological soundness, based on the farmer's managerial ability and operational cost-effectiveness, market and pricing structure, trade-offs with respect to maintenance of environmental quality, prevailing infrastructure, and policy environment. Of course, the measures that ensure long-term sustainability may be unattractive in the short term to the farmer, in which case certain technology characteristics or state policy may be used to achieve the desired objective (Okigbo 1984).

In a sustainable farming system, despite the vicissitudes of the weather, climate, political and socio-economic conditions causing perturbations in the yield curve, the overall trend does not show a decline in the long term. This paper is devoted to a review of the main characteristics of the farming systems of tropical Africa, extent of their sustainability, changes they are undergoing under impacts of different human activities and a host of other factors, reasons why they are becoming unsustainable and what needs to be done to render them more sustainable now and in the future. Finally, based on an understanding of the causes of unsustainability of the existing farming systems, the ingredients that must be wrought into farming systems during the design stage are considered.

Farming systems of tropical Africa and their sustainability under changing conditions

Farming systems in tropical Africa consist of an amalgam of crops and animals managed in various production systems with their component cultural practices and technologies made up of varying mixes of traditional and introduced elements adapted to the requirements of different ecological zones and peoples of diverse cultures. As in other parts of the world, these systems are culminations of several millennia of experimentation which gave rise to extensive production systems such as shifting cultivation and nomadic herding - sustainable systems that were economically viable, ecologically sound and culturally acceptable under the then prevailing low population densities. With increasing population pressure these gave rise to more intensive fallow systems. The various characteristics of these farming systems are listed below:

• small farm size with over 80 per cent of the farms between 0 and 5 ha;
• farming predominantly for subsistence but increasingly becoming commercial with from less than 10 per cent to over 90 per cent of produce offered for sale in local markets or for export;
• widespread reliance on simple tools and manual labour with minimal use of animals and/or machinery for work;
• marked division of labour between the sexes;
• most farming activities and production systems very closely related to prevailing rainfall regimes;
• acute labour shortage especially during peak periods of main farm operations such as clearing, planting, weeding, etc.
• widespread practice of slash and burn clearance systems;
• widespread reliance on fallowing and nutrient cycling by vegetation for maintenance of soil fertility lost during the cultivation phase or as a result of prolonged and more intensive cultivation;
• lack of credit and hence limited use of costly inputs such as fertilizers, soil amendments, pesticides, farm machinery, etc., which could significantly increase areas under cultivation and total productivity;
• in most traditional farming systems crop production may be practiced hand in hand with livestock or animal rearing, but the extent of integration varies from one farm to another and the number and species of animals kept depend on the presence or absence of diseases such as trypanosomiasis;
• political instability and ethnic strife in some countries, making production impossible or harvesting risky (Okigbo 1985).

The various farming systems consist broadly of traditional (e.g. bush fallow and compound farms) systems, transitional systems (e.g. smallholder cocoa and coffee plantations) and modern farming systems and their local adaptations, such as large-scale plantations, ranches, poultry farming and market gardening. The details of these typologies need not concern us here. What is of concern is that the farming systems are not static. They are changing as a result of changes in the environment, both natural and socio-economic. Some of these changes have rendered the traditional farming systems unsustainable and somewhat outmoded. A few examples of the changes and their effects on sustainability are presented hereunder:

Change	Effect on Sustainability
Introduction of Asian and American crops	Positive and negative
Population explosion	Negative
Commercialization of agriculture	Largely negative for low resource farmers
Mechanization	Largely negative, sometimes positive
Agricultural chemicals	Largely negative unless strictly controlled
Fertilizer use	Negative and positive
European settlement	Negative and positive

The manner in which changes affect sustainability can be illustrated with two or more of these examples. For instance, the introduction of Asian and American crops can be regarded as contributing to the increase of biodiversity and therefore contributing to increasing stability of production and biodiversity. But it is also true that the production of the introduced crops has often been promoted at the expense of the indigenous food crops, some of which are so neglected that they are not much being grown and a considerable degree of biodiversity has been lost. Population explosion has considerably increased pressures on land, resulting in intensification of farming associated with a drastic shortening of the period of fallowing from about 10 years or more to only 2 years or less. Use of agricultural chemicals has different impacts on the environment. Where reasonable amounts of chemicals are appropriately used, the effects are largely beneficial. Where, for example, no fertilizers are used and farming is intensified, the nutrients are depleted and yields drop as soils become degraded. But where excess amounts of farm chemicals are applied the environment may become polluted and unsustainability is the result.

Causes of Unsustainability in Agriculture of Developing Countries in Africa

The intensification of agricultural production as a result of increasing population pressure, intensification of farming, overgrazing and conversion of land to several uses that were not tested in the evolution of farming systems in Africa, have resulted in several undesirable changes in the environment with adverse effects on agricultural production. Figure 4.1 shows that with intensification of farming due to population and other pressures the following changes occur:

Figure 4.1a Why and How Certain Systems Become Non-viable (Source: FAO 1991a)

1.

• intensification of land use for food production
• shortened fallow periods
• reduced capacity for soil regeneration
• decline in soil fertility
• reduced and declining yields
• food shortages
• food imports

Figure 4.1b Why and How Certain Systems Become Non-viable (Source: FAO 1991a)

2.

• use of marginal sloping lands
• overgrazing and overstocking
• loss of vegetative cover
• exposure to wind
• exposure to rainfall
• increased surface run-off
• soil erosion
• landslides and downstream flooding
• reduced aquifer
• water shortages

Table 4.1 Inputs of Technologies Used in Traditional and "Modern" Conventional Farming Systems

	Traditional agriculture	Modern agriculture
Land	Small (<1-5 ha)	Large (10-100 ha or more)
Tools	Simple: fire, axe, hoe, digging sticks, machete	Complex: tractors and imple meets, threshers, combine harvesters, etc.
Crops	Many species (5-80), land races, no genetic improvement, wide genetic base	Few species (1-3), improved narrow genetic base
Animals	Several species (2-5)	Usually 1 or 2 species
Labour	Manual, human energy, or animal power	Mechanical, petroleum fuels, electrical energy
Soil fertility maintenance	Fallows, ash, organic manures	Inorganic fertilizers, sometimes manures, soil amendments, e.g. lime and gypsum
Weed control	Manual, cultural	Mechanical, chemicals (herbicides and petroleum-based products)
Pest and disease management	Physical/cultural	Mainly mechanical, chemicals, insecticides, fungicides, bactericides, nematocides, rodenticides
Crop management	Manual	Growth regulators for defoliation, control of flowering, fruit drop, etc.
Harvesting	Manual or with simple tools	Mechanical, tractors plus implements: pickers, balers, threshers, combine harvesters
Post-harvest handling and drying	Simple sun-drying and over fires	Mechanical forced-air artificial drying using petroleum fuels, sometimes refrigeration

Source: Okigbo (1988).

3.

• increased fuel needs
• deforestation
• unregulated logging
• loss of fauna and flora habitat
• extinction of genetic species and loss in biodiversity

The changes that take place under intensive agriculture are the same as those that occur under shifting cultivation except that the inputs used vary.

Table 4.1 shows the differences in practices and inputs used in traditional agri culture as compared to those used in "modern" intensive agriculture. Note that while simple hand tools used in traditional agriculture do not cause compaction, heavy machinery used in intensive agriculture does and this, in turn, causes structural deterioration, poor drainage and waterlogging.

Case-study of Changes Causing Environmental Degradation and Reduced Productivity in Southern Nigeria

Lal and Okigbo (1990) conducted an assessment of soil degradation in the southern states of Nigeria and identified factors that cause environmental degradation in the humid tropics of southern Nigeria and in the humid tropical African environment.

The main change in traditional farming systems is that of intensification of farming and the shortening of the periods of fallow. It was found that changes that occur in the soil are physical, chemical and biological, and there were changes of a socio-economic character also. If these changes are identified, it is possible for us to incorporate into the production system practices, technologies, etc., which will prevent adverse changes that threaten sustainability.

The main causes of soil degradation encountered were:

• fire and burning of vegetation especially on roadsides and adjacent areas covered by Siam weed, Chromolaena odorata; deforestation and land clearing;
• intensification of farming and shortening of fallow periods;
• low input agriculture - no fertilizers applied to food crops under continuous cultivation;
• accelerated erosion;
• construction of roads, buildings and industrial infrastructure.

The most serious soil degradation occurred in areas where fallow periods are minimal or non-existent. But the effects of long periods of cultivation often result from various practices ranging from clearing and cultivation to subsequent cropping. Symptoms of soil degradation observed were:

Physical

• surface seal formation, crusting and compaction of surface horizon;
• reduction of infiltration rate;
• reduced water-holding capacity; increased bulk density;
• increased susceptibility to raindrop impact and soil splash;
• leaching out of clay and colloidal fraction in the subsoil.

Chemical

• acidification resulting from depletion of bases;
• decline in pH higher in coarse textured soils;
• increase in exchangeable Al and H;
• reduction in soil organic matter and organic carbon;
• reduction in total and available N and P;
• nutrient depletion culminating in multiple nutrient deficiencies.

Biological

• changes in number and composition of soil fauna and flora;
• overall decline in soil fauna under cultivation;
• seasonal migration of fauna from top to subsoil;
• clearing and burning resulting in a tendency of pests and diseases to increase, especially where no rotation is practiced;
• deforestation reported to cause decline in populations of parasitic nematodes after clearing;
• chemicals, herbicides, insecticides which adversely and differentially affect soil fauna;
• cropping or cultivation which produces differential effects in different situations;
• cultural practices such as mulching on surface which encourage earthworm activity;
• changes in weed species and composition related to commodity produced and practices;
• burning, which may lower earthworm activity.

Yield Reduction

• this may be regarded as a biological effect of soil degradation;
• reduction in yield as a result of degradation varies with soil fertility and structure.

Commodities and Production System Changes

• increase in areas under cassava and decrease in those of maize and yams, which may be more due to cassava being adapted to degraded soils than to people preferring to eat and grow cassava.

Sustainable Farming Systems

The assessment of soil degradation in southern Nigeria also resulted in identification of the following as the main sustainable farming systems currently practiced or emerging:

• home gardens or compound farms;
• planted fallow systems;
• alley cropping and related agroforestry systems;
• wetland rice;
• market gardens and some tree crop plantations.

It is obvious from the above that many of the traditional and arable crop farms, especially those that are highly commercialized and on which most of our fertilizers are used, are not sustainable. It is on the basis of the above considerations that the elements of sustainable farming systems to be considered in designing sustainable farming systems will be based in addition to issues discussed in SCA (1991) and Lal and Okigbo (1990).

Ingredients of sustainable farming systems and issues to be considered in the design of these systems

The overall objective of agricultural or farming system design and management is the creation of environmental conditions that remain favourable for crop and animal production or even increase their productivity in perpetuity while at the same time minimizing adverse impacts on the resource base or, where possible, enhancing it. Consequently, it is necessary in any effort at designing sustainable farming systems to begin with:

• reviewing the characteristics of environmental resources and ways of managing and manipulating them to achieve specific results;
• selection characteristics of inputs, practices and technologies that in tropical Africa can be used to render outmoded and new farming systems sustainable. Table 4.2 lists resource characteristics, practices that render them unsustainable and restorative measures for dealing with the undesirable changes.

In addition to the various causes of unsustainability indicated in figure 4.1, or the characteristics of resources and inputs which may, in association with certain practices cause unsustainability for which remedies are given (table 4.2), there are overall general issues that need to be taken into account in designing sustainable agricultural systems. These include:

• The need for low resource farmers in developing countries to minimize use of external inputs such as fertilizers and herbicides by increasing reliance on internal inputs, because of lack of credit and foreign exchange. In this regard, we can also regard the buying of stakes from neighbours or the local market as purchase of an external input which can be internalized by using agroforestry systems such as alley cropping, which supplies stakes and even fuelwood that is home grown.
• Increase reliance on biological processes such as biological nitrogen fixation, nutrient cycling mechanisms and mycorrhizal phosphate nutrition for minimizing the amount and cost of fertilizer use.
• The fact that the use of fertilizers (organic or synthetic) in tropical agriculture is imperative and that the only options we have are how to optimize the use and increase the efficiency of fertilizer use.
• In some situations we may resort to the growing of crops (e.g. cassava) that are adapted to soil acidity wherever possible rather than always resorting to liming in dealing with soil acidity.
• Use of integrated approaches in agriculture systems, e.g. integrated watershed development;
  • integrated pest, disease and weed management (IPM);
  • integrated field crop and shrub/tree (ligneous species) production systems agroforestry;
  • integrated crops, trees, pastures/animal systems (agrosilvopasture);
  • mixed cropping systems and rotational sequences rather than monoculture;
  • integrated land use planning; - integration of traditional, "modern" and emerging technologies;
  • minimal and strategic use of pesticides and chemicals associated with a spectrum of compatible physical, cultural and biological methods;
  • combination of manual and appropriate mechanization systems;
  • need for intensification of production and use of appropriate technologies under increasing population pressures, since shifting cultivation and related fallow systems cannot meet the demands of higher carrying capacity (table 4.3);
  • elimination of gender bias in technology development and extension;
  • need for adequate level of human resources development and institutional capacities in research and development to keep up with changing needs of farmers and changing circumstances;
  • research of sufficient scope on research stations and on the farm level with increasing farmer participation;
  • balance between production research and post-harvest-phase research and development necessary for successful agribusiness growth;
  • need for policy umbrella in support of sustainable agriculture and one which ensures compatibility among agriculture, forestry, fisheries, etc., in order to render practices more environmentally friendly;
  • need for suitable infrastructure in support of sustainable agriculture, including roads from farms to markets and effective pricing and market structure.

Table 4.2 Selected Sustainable Agriculture Resource Input Features and Practices, Effects and Remedies

Resource characteristic	Practice or factor contributing to unsustainability	Ameliorating or restoration technology or practice
1. Land tenure	Lack of security of tenure	Secure land permanently or for period that commodity is in field
2. Land use	Absence of plan or agreement on plan	Start as early as possible to get authorities thinking of and evolving plan
	Land in dispute	Early settlement before use, especially for perennial crops
3. Vegetation management	Use of heavy mechanical equipment	Selective mechanization, e.g. use of shear blade and partially mechanized clearing; avoid very heavy equipment
		Avoid burning large amount of dry biomass for long periods at high temperatures
	Overgrazing	Relate stocking rate to pasture condition; use rotational grazing and fence partitioning
		Relate all land use to the capability
4. Soils:
(a) Structural damage or decline	Fire pasture	Early burning of limited dry biomass
	Excessive or mechanical cultivation	Develop appropriate fire management for specific land use requirement
	Fallowing and bare soil; overgrazing and loss of cover; machinery and animal traffic	Ensure adequate cover by vegetation or mulch
		Rotational grazing and cover management
		Relate tillage and stocking rate to pasture condition and soil type
(b) Acidification nutrient loss	No lime used on acid soil; use of acidifying fertilizers	Lime application if possible and
		Use most appropriate recommendations
		Use of resistant or acid tolerant varieties
(c) Erosion	Removal of vegetation cover and exposure of soil	Retention of vegetation cover by stocking adjustment, good pasture and/or wildlife management, stubble mulch - rough surface retention
	Overgrazing
	Degradation resulting from poor cultivation technique	Use of reduced or minimal tillage, deep ripping, pasture rotation and measures to rejuvenate fragile soil
		Use of wind-breaks and alley cropping
	Adoption of land use that is not compatible or does not match capability of land	Improvement of capability assessment and better matching of use to it
5. Fire management	Uncontrolled use of fire in clearing, hunting, pasture management, etc.	Controlled use of fire; early burning in pasture management and to maintain desired species composition
6. Water quality	Inadequate drainage, waste and effluent water disposal	Improved engineering works to carry drainage water and effluent from animal housing; provision of sanitary inspection to enforce laws
	Contamination of surface and groundwater by fertilizer and pesticides	This is not of common occurrence and is limited to a few large-scale "modern" farms
		Care in use of pesticides near open water
		Measures to minimize access of chemicals to groundwater
		Use suitable fertilizer type and method of application to increase uptake
		Apply fertilizer in amounts needed by crops as determined by analysis
	Sediment and salt run-off into surface water	Better management to minimize soil erosion and salinity
7. Soil salinity, water- logging (irrigated agriculture)	Inefficient/excessive water use by flooding, too frequent irrigation, low infiltration	Improved water scheduling
		Conjunctive reuse of groundwater
		Drainage and gypsum to improve infiltration
	Inadequate/deteriorating infrastructure	Improved water distribution networks
	Poor site selection for irrigation areas	Soil selection should be consistent with soil and land capability
8. Soil salinity (dryland)	Excessive clearing of deep-rooted perennials causing rise in groundwater levels	Identification and revegetation of recharge areas
		Strategic tree and shrub planting/management
		Use of deep rooted perennials wherever possible
9. Use of monoculture crops	Reliance on a single crop without rotation or use of row crops	Better to use tested and row rotations
		Use mixed crop sequences rather than just row sole crops
10. Pesticide residues resistance	Overreliance and persistent use of pesticides	Use of integrated pest and management
	Overreliance on chemical control of crop weeds	Biological control of pests
		Selection of genetically resistant species
		Low pesticide use farming
		Use of biodegradable pesticides
		Use of rotations to reduce pest, weed or pathogen infestation

Sources: Adapted from SCA (1991) and Lal and Okigbo (1990).

Table 4.3 Yield in Gram Equivalents and Percentage of Crop Land for Various Levels of Production Inputs in the World

Farming system or technology input level	Yield t/ha	Crop land (%)	Average area of arable land needed (ha/caput)
Shifting cultivation	<0.1	2	2.6
Low traditional	0.8	28	1.2
Moderate traditional	1.2	35	0.6
Improved traditional	2	10	0.17
Moderate technological	3	10	0.11
High technological	5	10	0.08
Specialized technological	7	5	0.05

Source: FAO (1991b).

Sectorial interface requirements

However appropriate and realistic the design of a sustainable farming system may be, it is necessary to ensure that threats to it from other sectors are eliminated or significantly minimized. For example, poor road construction could result in flooding, eutrophication and erosion, all of which can seriously damage farm land and even fish-ponds or stream fisheries. A related example is a policy issue such as structural adjustment aimed at increasing export earnings or at reducing debt burdens. This may result in a lot of forest areas being cleared for the commercial row-growing crops, which will expose the soil to erosion. Similarly, removal of subsidies may result in farmers not using fertilizers which, in turn, will result in environmental degradation. Therefore, designing sustainable agricultural or farming systems and adhering to the design alone will not ensure sustainability unless policies, strategies, technologies, systems and components of resource management, and input/technology use in other sectors such as forestry, fisheries, animal production, manufacturing industries, tourism and management of nature reserves and trade are designed to ensure sustainability in development devoid of adverse impacts on the other sectors. A few examples of sectoral activities in one sector which affect other sectors are presented below:

Uncontrolled expansion of agricultural land reduces land available for reserves, forestry and other multiple land use requirements.

• Unplanned land use and expansion of agricultural land may result in reduc ing land available for pastures, culminating in overgrazing.
• Deforestation and inappropriate logging practices could cause erosion, landslides, siltation of streams, etc.
• Poorly managed industrial expansion could result in chemical pollution of the air and streams.
• Unregulated hunting and collection of trophies for sale to tourists or for export could result in loss of biodiversity and in ecosystem deterioration.
• Lack of family planning and uncontrolled population growth causes increased pressures on land resources, shortening the duration of periods of fallow and resulting in poor vegetation cover and erosion.

All these call for not only integrated land use planning but also adoption of a holistic approach in development planning, in policy formulation and in selecting strategies and the execution of development programmes. The earlier we pay attention to these, the better.

Conclusions and recommendations

In designing sustainable agricultural production systems, it is necessary to give due consideration to the characteristics of various resources used in production, the ways they are managed or manipulated in the production process and the technologies and practices which render the resultant production system unsustainable. Unsustainability results when impacts of practices and technologies used are economically not feasible and sometimes also culturally unacceptable. In addition to the selection of methods for manipulation of resources and use of practices and technologies which ensure sustainability, there is a need to ensure that other sectoral activities do not render the farming systems unsustainable. At the same time measures must be taken to ensure that various accelerators of agricultural development and factors which ensure enabling the environment for agricultural production for a majority of farmers are present. These include:

• effective marketing and pricing system for agricultural produce;
• continuous and systematic research that generates new agricultural practices and technology;
• presence of adequate incentive for farmers to increase output or for operators of agricultural services to perform their tasks efficiently;
• effective transportation and communication system that reaches most farms; and
• local availability at reasonable prices through manufacturing or importation of equipment and farm inputs needed by farmers (Mosher 19 70).

The effectiveness of these are accelerated by:

• presence of adequate educational and training facilities for agricultural technicians and experts in supporting services;
• effective input distribution mechanisms;
• opportunities for farmer group action through cooperatives and similar organizations;
• means for improving and expanding agricultural land; and
• mechanism for planning and directing agricultural development programmes as integral components of overall economic development (Mosher 1970).

Recommendations on Principles for Designing Sustainable Farming Systems in Tropical Africa

In designing sustainable farming systems there are five principles or objectives that should be aimed at, namely:

1. maintenance or enhancement of farm productivity in the long term;
2. amelioration, minimization or avoidance of adverse impacts on natural resource base for agriculture and associated ecosystem;
3. minimization of residues from chemicals in agriculture or of adverse effects of practices;
4. maximization of the net social benefit derived from farming, which involves considering net social benefits of agriculture when positive and negative effects are considered and making such choices among alternatives as to maximize benefits by using certain production systems and practices; and
5. rendering farming systems sufficiently flexible to manage risks associated with vagaries of climate and markets (SCA 1991).

Guidelines for Designing Sustainable Systems

The main guidelines to follow in designing sustainable agricultural systems are to:

• gather and obtain all available information on the environmental resources (climate, land/soil, water, diseases, pests, ecosystems, etc.);
• ensure that the land use capability of the area to be farmed or used is well known;
• use land clearing methods that cause minimal disruption of soil structure;
• ensure the matching or the compatibility of the land use with the land use capability and, where possible, on the micro level, relate land use to the characteristics of the toposequence in the watershed;
• give due consideration to prevailing environmental conditions in choosing the commodities to be produced, varieties to be produced and the timing of sequences of operations and nature of operations to be performed, from clearing to harvesting and storage (table 4.4);
• give due consideration to the probable environmental impacts of the practice or technology associated with each operation to be performed, and the choice to be made of the option with minimal adverse environmental impact;
• minimize overreliance on external inputs (i.e. inputs obtained from outside the farm ecosystem);
• as much as possible, use integrated approaches such as:
  • integration of traditional and modern or emerging technologies, e.g., use of partially manual and partially mechanical clearing;
  • integrated land use planning;
  • integrated watershed development;
  • integration of biological (nitrogen fixation) and chemical fertilizers and nutrient cycling;
  • integrated pest, disease and weed management, e.g., use of intercropping and manual methods in weed control;
  • integrated production systems, e.g. arable crops and woody tree/shrub agroforestry; crops and livestock (mixed farming); crops, shrubs and trees, pasture and livestock (agrosilvopasture); crops, animals and fish;
• as much as possible use biological approaches and processes while minimizing reliance on chemical and synthetic methods in soil fertility management, pest management, etc.;

• relate the choice of commodities to be produced and their characteristics to the post-harvest objectives and uses, e.g. storage, processing, planting, etc.;
• diversify production either through mixed cropping/intercropping, rotations of sole cropping, or mixed cropping, but also bearing in mind that row cropping is soil degradative;
• make as much effort as possible in all operations in soil management to keep the soil covered with crop residues and to maintain reasonable levels of soil organic matter;
• give fire management and control serious consideration in clearing, hunting, pasture management and in dealing with accidental fires;
• control soil surface cover in livestock management by control of stocking rate or by controlled grazing;
• intensify production in the wake of rapid population growth and decline in available land, introduce land saving technologies to increase carrying capacity, and promote family planning;
• improve sustainable agricultural design through monitoring of impacts of various practices and technologies in addition to changes in the resource base under different land use systems; refinement and changes should be backed up with research of sufficient scope and environmental monitoring activities based on standardized and reliable methods.

Table 4.4 Different Operations Performed during Different Stages of Crop Production and Utilization and Extent of Likely Erosion Hazard Involved

Operations at different stages of crop production and utilization	Extent of possible erosion hazardsa
Clearing	Very high
Land development	High
Tillage and pre-planting cultivations	High
Planting	Low
Subsequent soil management	Low
Water management	Low-high
Fertilization	Low
Weeds, pest and disease management	Negligible-high
Harvesting	Medium-high
Primary processing (e.g., shelling, winnowing)	Negligible
Drying	None
Storage	None
Processing	None
Packaging	None
Preparation	None
Consumption	None
Waste disposal	Low-medium

Source: Okigbo (1985,1993).
a. Extent of erosion hazard depends on interaction of operations with environment and other factors.

The above principles and guidelines are by no means exhaustive but they are illustrative of the principles and procedures involved in determining: the management schedules; the manipulations of resources; and the input orches tration in amounts, sequences and timing in order to satisfy objectives in pro ducing desirable levels of food, fibre, and other products or otherwise satisfy ing the farmers' objectives. The design can only operate with success where there is an appropriate policy umbrella in the presence of all the accelerators of agricultural development and compatibility among all sectors.

References

BOA/NRC. 1989. Alternative Agriculture. Committee on the Role of Alternative Farming Methods in Modern Production Agriculture, Board on Agriculture, National Research Council (BOA/NRC). Washington, DC: National Academy Press.

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