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Suggested approaches to sustainable production
Wisdom from farmers' production systems
Systems that have a high potential for sustainability mimic natural ecosystems and ensure continuity in supply of some important resources for plant growth, e.g. vegetation, nutrients, and water, and prevent their losses from soils by erosion. These systems are those practiced by farmers whose habitats scientists are trying to understand. Biological control is one of the benefits of maintaining the ecological diversity of plants and animals.
Therefore, approaches that incorporate farmers' relevant knowledge and experience into designs for improvement will most likely be sustainable, as has been shown in the examples of mimicking natural ecosystems. The Association of Farming Systems Research and Extension (Norman 1982; Okigbo 1989; Hildebrand 1990) emphasizes approaches requiring a thorough diagnosis of existing systems and conditions before designing any improvements and experimenting and testing them in farmers' fields. Improvements will definitely be necessary because the existing farmers' systems, developed in a given set of conditions (such as shifting cultivation at low population densities with bush path highways), break down when practiced in situations for which they are not meant.
Such improvements should gradually build up from existing systems (Hildebrand 1990) instead of replacing them completely. When replacement is the adopted approach, the system breaks down and frustration occurs, as was noted by Nye and Greenland (1960: v), who reviewed the effectiveness of technologies generated in Europe and applied in tropical Africa and observed that "after a quarter of a century of experiment in the African tropics, we have failed to introduce to the forest regions any method of staple food production superior to the system of natural fallowing used in shifting cultivation." The current research emphasis on multiple cropping, agro-forestry, including alley systems, and mulching of various kinds to produce plant cover for the soil represents a refinement of farmers' shifting cultivation to fit emerging situations.
Requirements of new technologies
To be sustainable, new technologies should address the whole farm and interactions in plant production systems (Hildebrand 1990). Donor interest in Farming Systems Research and Extension (FSR/E) may no longer be strong, as was shown by the funding problems of FSR/E and the complete absence of donors during a recent international meeting (AFSR/E annual meeting, East Lansing, Michigan, September 1992). Yet sustainability and FSR/E are compatible concepts because whole farm situations and interactions of their components need to be understood. An implication is that the days of neatly laid out field experiments with one or two factors and high environmental controls may be over (Federer, personal communication, 1991). High levels of variability are to be expected at farm level and more attention to interactions than to the main effects may pay off in understanding farmers' reel situations and problems.
Sub-Saharan Africa and water problems
At optimum water supply and disease and pest control, Linnemann and his co-workers (1979) showed that the potential annual grain production in Sub-Saharan Africa was about 10 billion metric tons. Burningh et al. (1975) estimated that the potential agricultural area in Africa in 1965-1973 was 23 per cent of Africa's total land area. Out of this area, only 6 per cent is cultivated, with a yield of 1,000 kg/ha, i.e. about 10 per cent of the 10,000 kg/ha potential. Burningh et al. (1975) concluded that the problem is one more of social and economic than of biophysical potential. Nevertheless, biophysical factors are of great importance if this potential is not exploited in a sustainable manner. Many of the improved crop varieties currently in use in Sub-Saharan Africa do not yield as much as they would if adequate water were available. In the humid zones of West and Central Africa, water distribution is such that crops are oversupplied for certain months of the year and undersupplied in others. A lack of water is also noted in some areas in the humid zone because of poor distribution, which often reduces the duration of crop growth (de Wit et al. 1979). Eastin et al. (1969) have shown that moisture stress adversely affects inflorescence development and grain filling and leads to inefficient use of nutrients. With stress, either fertilization is inhibited or grains that are already fertilized abort. Irrespective of breeders' expected yields, crop yields decline because of the reduced duration of growth attributed to the moisture stress of cereal grains (de Wit et al. 1979) or of roots and tubers (Lawson 1985). An urgent technical input is to ensure timely water distribution for plant growth. Even for cassava, which is tolerant of moisture stress, storage root yield is decreased during the dry season months of the transitional zone of West Africa (Lawson 1985).
Implications for plant breeding
For most plant breeders, favourable yields are based on high economic yields and the high harvest indices of improved varieties per unit of space and time. Varieties are usually evaluated in monocropping systems using the recommended levels of fertilizer and pest control. Evidence now available suggests that these values need to be reassessed in plant production systems aimed at sustainable development. To conserve soil resources, there is a need to conserve biological life: living organisms can be adversely affected by pesticides and other chemicals; ploughing accelerates soil and nutrient losses owing to erosion and leaching; good harvests at the expense of leaves, stems, and other plant parts may be detrimental because the additional vegetative materials needed for mulching the soil are lost. In highly vegetative plants, nutrients that could have been leached off or volatilized from soil may be held in plant parts and later released during decomposition.
The age of sustainability calls for a modification of breeding objectives. It may require new approaches in testing improved varieties and a re-examination of selection procedures. A trait lost may be hard to recover at later stages in a breeding cycle. An example is cassava, whose shoot and root yields may be reduced significantly by spraying cowpea intercropped with the cassava as compared with unsprayed cowpea and cassava (fig. 11.3). Without a systems approach, this interaction may be missed because cowpea protected from insect attack by spraying yielded higher than when unsprayed. Thus the limited objective of increasing cowpea yield is achieved but the broader objectives of ecological balance and of increasing total yield are not attained.
The lack of involvement of users in the development of technologies can lead to non-acceptance of the results. The success of intensive agricultural development in Taiwan is mainly the result of farmer pressure on researchers. In a study of 150 selected projects by US firms, Merrill-Sands (1992) reported a failure rate of about 67 per cent and a success rate of 15 per cent in cases where user influence on the products developed was characterized by hostility and distrust. The exact opposite, 67 per cent success and 14 per cent failure, was observed when client pressure or demand-pull led to product development. The high rate of cassava (TMS 30572) and sweet potato (TIB1) adoption in West Africa, particularly in Nigeria and Cameroon, respectively, appears to relate more to their development in unfertilized soil, which may have simulated the farmers' soil conditions, than to any other factor.
Virtually all the crops that generate export income, such as cocoa, coffee, and oil-palm, are declining in importance in the world market. For increased export earnings, Sub-Saharan Africa may need to produce specialized products in which it commands some advantages. The luxuriant vegetation, fruits, and nuts in African landscapes contain important oils, some protein, lubricants, and condiments (Okigbo 1980). Their potential benefits to Sub-Saharan Africa and in the world market remain unknown. For example, the cash income derived from some protected isolated stands of trees (Elaeis guineensis, Irvingia gabonensis, Raphia spp., and Bytyrospermum paraatoxum) in southern Nigeria in 1980 was at least 2.3 naira per tree per day or over 10 naira (US$0.5) at 1980 rates (Okafor 1980). Multiplication and improvement of important plants can be effected by biotechnology, which also has considerable potential in health care (medicine, nutrition) and in the chemical and cosmetic industries. New, improved crops with desirable characteristics, as well as cheap pest and disease control measures, are other potential benefits of biotechnology that Sub-Saharan Africa may miss out on if action is not taken to utilize existing facilities and create new ones if necessary.
Enhancing women's productivity
To attain sustainable development arising from the increased agricultural productivity of Sub-Saharan Africa, the human resource potentials of both men and women should be developed. Concrete efforts should be geared towards enhancing the productivity of all those involved in food crop production. As women are now responsible for the bulk of the food crops produced on small farm holdings, the emphasis should be on providing them with access to the resources and inputs necessary to increase their productive capacity (specific resources have already been discussed).
This paper has discussed the potential for developing sustainable plant production systems for Sub-Saharan Africa. Only a small fraction of the biophysical potential for crop production in Sub-Saharan Africa is exploited. Although the constraints on plant production are many, they can be reduced to three main focus areas: rainfall, soil related issues, and human resource management.
There is a need to develop methods of conserving the available water and to invest in water storage, particularly for those semi-arid areas of Sub-Saharan Africa where there is a serious lack of moisture. This requires investment, but approaches currently used by farmers (stone and plant barriers, surface catchments, tied ridges) need to be improved and inland valleys should be exploited. All these require more research. Agro-forestry species tolerant of acid soils and adapted to the semi-arid zone should be identified and evaluated for their usefulness in alley or related systems.
In the humid forests, the Guinea and moist savannas, and the highlands of eastern Africa, agro-forestry systems, multiple-cropping systems, and surface mulching using appropriate live mulch species need to be studied further. These have elements of sustainability that require additional research prior to their wide application. Of particular importance is the interaction of live mulches with soil moisture, especially in dry areas. With the increasing cost of imported chemicals, fertilizer research should be redirected to determining the minimum levels required for sustained yields and to augment nutrients generated through biological activity.
Holistic approaches to technological developments pay off in terms of their ease of adoption. Step-wise approaches to technology development, using farmers' current systems as the base, appear to have merits for the small-scale farmers of Sub-Saharan Africa. Research into whole farm systems requires a revision of the operational approach. Farm-level research is very complex and involves many interactions; it therefore requires multidisciplinary team efforts. Less rigorous but useful analytic approaches should be devised for interpreting data. Most of the farmers in Sub-Saharan Africa are women. Their neglect in resource allocation should be redressed by gearing efforts towards enhancing their productivity.
Finally, biotechnology may be the cheapest hope if it is applied to resolving some of the constraints, for example through multiplication of improved varieties, particularly clonal materials (Zok and Nyochembeng 1992), conservation of wild and exotic species, and breeding for disease and pest resistance and much other gene-related research. Regional laboratories to handle this specialized work need to be created, staffed, and funded for the mutual benefit of the countries of Sub-Saharan Africa, and special management procedures devised for the laboratories. Some existing laboratories - for example, the Jay P. Johnson Biotechnology Laboratory, Ekona, Cameroon, and the International Institute of Tropical Agriculture Biotechnology Laboratory at Ibadan, Nigeria - could be expanded to serve parts of the region, and the possibility of establishing others needs to be evaluated. Courses in biotechnology to acquaint researchers and policy makers with its potential (similar to the one designed by Ferguson et al. 1992) could be modified to fit Sub-Saharan African conditions and videotaped for popular use. Such a course could remind researchers and policy makers of the usefulness of cell biology in agriculture and of the basic training requirements.
1. Sub-Saharan Africa has more than enough biophysical resources to produce more food and other plants at sustained levels.
2. Research should be directed to exploiting existing biological complementarities in a holistic manner. Inorganic inputs should be used to complement, not to replace, biological inputs.
3. Post-harvest losses constitute a major source of productivity loss in Sub-Saharan Africa.
4. Since most crops in Sub-Saharan Africa are produced by women, gender issues are very important and should be addressed in developing plant production and utilization technologies.
5. Small-scale farming is not necessarily bad, and increased production does not necessarily call for increased farm size.
6. Given the adverse effects of mechanization in terms of the degradation of soil resources in the tropics, one must be cautious about introducing mechanization, especially because it is not a panacea for increased production and yield (see 5).
7. With serious effort and resource commitment, the perennial problem as regards water for crop production and for human needs could be resolved. This is an area in which investment probably could be made to pay, bearing in mind that some parts of California, Nevada, and Israel, for example, which produce surplus food, are in deserts.
8. Continued conservation of species diversity, especially some wild species, is advocated. What is wild today may find a use tomorrow.
9. The potential of biotechnology needs to be exploited through organized groupings of Sub-Saharan African regions to invest in gene-related research.
We are grateful to Mr. George Gamze for help in reproducing this paper. To Mrs. C. N. F. Poubom, Mr. A. E. Efite, and Dr. Charles Yamoah we give thanks for proofreading the manuscript and making useful suggestions. In spite of these contributions, all responsibility for the contents of this paper is that of the authors.
Aina, P. O., R. Lal, and G. S. Taylor. 1977. Soil crop management in relation to soil erosion in the rainforest of Western Nigeria. In: Soil Erosion Prediction and Control. Soil Conservation Society of America, Special Publication 21: 75-84.
Akobundu, 1. O. 1980. Live mulch: A new approach to weed control and crop production in the tropics. In: British Crop Protection Conference: Proceedings of the 1980 British Weed Control Conference, 17-20 November, Brighton, England. Oxford: ARC Weed Research Organization, pp. 377-382.
Babalola, S. O. and C. Dennis 1988. Returns to women's labour in cash crop production: Tobacco in the Igboho, Oyo State, Nigeria. In: J. Davidson (ed.), Agriculture, Women and Land: The African ExpeHence. Boulder and London: Westview Press, pp. 79 89.
Boserup, E. 1970. Woman's Role in Economic Development. New York: St. Martin's Press.
Bruijn, G. H. de and L. O. Fresco. 1989. The importance of cassava in world food production. Netherlands Journal of Agricultural Science 37: 21 34.
Bukh, J. 1979. The Village Woman in Ghana. Uppsala, Sweden: Scandinavian Institute of African Studies.
Burningh, P., H. D. J. Van Heemst, and G. J. Staring. 1975. Computation of the absolute maximum food production of the world. Department of Tropical Soil Science, Agricultural University, Wageningen, The Netherlands.
Carney, J. A. 1988. Struggles over land and crops in an irrigated rice scheme: The Gambia. In: J. Davison (ed.), Agriculture, Women and Land: The African Experience. Boulder and London: Westview Press, pp. 59-78.
Carr, J. S. 1989. Technology for Small-scale Farmers in Sub-Saharan Africa. Experience with Food Crop Production in Five Major Ecological Zones. Technieal paper no. 109. Washington D.C.: World Bank.
Charreau, C. 1970. Traditional Afriean agricultural systems and their improvement. FAO/IITA Conference on Shifting Cultivation, Ibadan, Nigeria, mimeo.
Cloud, K. 1985. Women's productivity in agricultural systems: Considerations for project design. In: C. Overholt, M. B. Anderson, K. Cloud, and J. E. Austin (eds.), Gender Roles in Development Projects. A case book. West Hartford, Conn.: Kumarian Press, pp. 17 56.
Collinson, M. 1987. Potential and practice in food production technology development: Eastern and southern Afriea. In: J. W. Mellor, C. L. Delgado, and M. J. Blackie (eds.), Accelerating Food Production in Sub-Saharan Africa. Baltimore, Md.: Johns Hopkins University Press.
Conway, G. R. 1985. Agroeeosystem analysis. Agricultural Administration 20: 1 25.
Dey, J. 1981. Gambian women: Unequal partners in riee development project? In: N. Nelson (ed.), African Women in the Development Process. London: Frank Cass, pp. 109-122.
Dover, M. and L. M. Talbot. 1987. To Feed the Earth: Agro-ecology for Sustainable Development. Washington D.C.: World Resources Institute.
D'Silva, C. and M. R. Raza. 1983. Equity considerations in planning and implementing rural development projects in Nigeria: An evaluation of the Funtua Project. In: B. L. Green Shield and M. A. Bellamy (eds.), Rural Development Growth and Inequity. IAAE Occasional Paper no. 3. Brookfield, Vt.: Ashgate Publishing, pp. 101-106.
Eastin. J. O., F. A. Haskins, C. Y. Sullivan, and C. H. M. Van Bavel. 1969. Physiological significance of internal water relations to crop yield. In: Physiological Aspects of Crop Yield. Madison, Wis.: Ameriean Soeiety of Agronomy and Crop Seience Society of America, pp. 53-85.
Ezumah, H. C. 1990. Maize genotypes for intercropping with cassava in Southern Nigeria. Yield responses. Discovery and Innovation (African Academy of Sciences) 2(2): 63 72.
Ezumah, H. C. and 1. E. G. Ikeorgu. 1993. Population and planting pattern effects on intercropped maize and cowpea. Journal of Agronomy and Crop Science 170: 187-194.
Ezumah, H. C. and T. L. Lawson. 1984. Maize plant "architecture". Important factor in intercropping with cassava. IITA Research Highlights. Ibadan, Nigeria: IITA, pp. 96-99.
---- 1990. Cassava and maize intercropping systems. 1. The effects of varieties and plant population. Journal of Agronomy and Crop Science 164: 334 342.
Ezumah, N. N. t990. Women in development: The role of lgbo rural women in agricultural production. Unpublished Ph.D. dissertation, University of Ibadan, lbadan, Nigeria.
FAO (Food and Agriculture Organization of the United Nations). 1978. Food Balance Sheets and Per Caprta Food Supplies. Rome: FAO.
---- 1982. Follow-up to WCARRD: The Role of Women in Agricultural Production. Rome: FAO.
Ferguson, N. H., P. E. Adams, and R. E. Franklin. 1992. A plant biotechnology course in distance delivery. Journal of Natural Resources and Life Sciences Education 21(2): 133-136.
Francis, C. A. 1989. Biological efficiencies in multiple cropping systems. In: Advances in Agronomy, vol. 42. New York: Academic Press.
Guyer, J. I. 1984. Naturalism in models of African production. Man 19: 371-388.
---- 1986. Women's role in development. In: R. J. Berg and J. S. Whitaker (eds.), Strategies for African Development. Berkeley: University of California Press, pp. 393 421.
Hahn, S. K., E. R. Terry, K. Leuschnor, I. O. Akobundu, C. Okali, and R. Lal. 1979. Cassava improvement in Africa. Field Crop Research 2: 193-226.
Harrison, P. 1987. The Greening of Africa. Harmondsworth, Middlesex: Penguin Books.
Herren, H. R. 1989. IITA/Africa-wide biological control programme: History, achievements and prospects for sustainable pest control in Africa. In: Seminar Report: integrated Pest Management, 5 6 Sept 1988. Sem, Asker, Norway: NORAGRIC, pp. 15-32.
Hildebrand, P. E. 1990. Agronomy's role in sustainable agriculture: Integrated farming systems. Journal of Production Agriculture 3(3): 285 288.
Hoogh, J. de, M. A. Keyser, H. Linneman, and H. D. J. van Heemst. 1976. Food for a growing world population: Some of the main findings of a study on the long-term prospects of the world food situation. European Review of Agricultural Economics 3: 459-499.
Hullugale, N. R. 1989. Effect of tied ridges and undersown Stylosanthes hamata (L) on soil properties and growth of maize in Sudan Savanna of Burkina Faso. Agriculture, Ecosystems and Environment 25(1): 39-51.
Hullugale, N. R. and H. C. Ezumah. 1991. Effects of cassava-based cropping systems on physico-chemical properties of soil and earthworm casts in a tropical Alfisol. Agriculture, Ecosystems and Environment 35: 55-63.
IITA. 1978, 1980, 1983, 1984, 1985, 1989, 1990. Annual Report. Ibadan, Nigeria: International Institute of Tropical Agriculture.
Ikeorgu, J. E. G. and H. C. Ezumah. 1991. Some analytical aspects of cassava/maize/ eguei/okra melon complex mixture: Soil temperature in relation to leaf area variation. Field Crops Research 27: 51-60.
Iuo, A. S. R. and H. C. Ezumah. 1992. Mixed root crop ecosystems in the wetter regions of Sub-Saharan Africa. In: C. J. Pearson (ed.), Field Crop Ecosystems of the World. Amsterdam: Elsevier Scientific Publishers, pp. 243-258.
Juo, A. S. R. and 1. A. Lowe (eds.) 1986. Wetland and Rice in Tropical Sub-Saharan Africa. Ibadan, Nigeria: IITA.
Kang, B. T. 1988. Nitrogen cycling in multiple cropping systems. In: J. R. Wilson (ed.), Advances in Nitrogen Cycling in Agricultural Ecosystems. Wallingford, UK: CAB International.
Kang, B. T. and A. S. R. Juo. 1981. Management of soil with low activity clays for food crop production in tropical Africa. In: Proc. International Soil Classification Workshop, Kigali, Rwanda. Technical Monograph, Soil Management Support Service, USDA, Washington D.C.
Kang, B. T. and G. F. Wilson. 1987. The development of alley cropping as a promising agroforestry technology. In: H. A. Stepler and P. K. R. Nair (eds.), Agroforestry: A Decade of Development. Nairobi: ICRAF, pp. 227--243.
Kowal, J. M. and A. H. Kassam. 1978. Agricultural Ecology of Savanna: A Study of West Africa. Oxford: Clarendon Press.
Kumar, S. K. 1988. Women's role and agricultural technology. In: J. W. Mellor, C. L. Delgado, and M. J. Blackie (eds.), Accelerating Food Production in Sub-Saharan Africa. Baltimore, Md.: Johns Hopkins University Press, pp. 135 147.
Lal, R. 1976. Soil Erosion Problems and an Alfisol in Western Nigeria and Their Control. IITA Monograph no. 1, Ibadan, Nigeria.
---- 1989. Conservation tillage for sustainable agriculture: Tropics vs temperate environments. In: Advances in Agronomy, vol. 42. New York: Academic Press, pp. 85 119.
Lal, R. and D. J. Greenland. 1978. Effect of conditioners and moisture content on temperature profile and infiltration rate of a vertisol. In: W. W. Emerson, R. D. Bound, and A. R. Dexter (eds.), Modification of Soil Structure. London: John Wiley, pp. 191-198.
Lawson, T. L. 1979. Agroclimatology section. IITA Annual Report.
---- 1985. Agro-climatic consideration for cassava cropping. IITA Research Highlights, 1981-1984. Ibadan, Nigeria: Farming Systems Program, IITA.
Lawson, T. L. and B. T. Kang. 1990. Yield of maize and cowpea in alley cropping systems in relation to available light. Agricultural and Forest Meteorology 528: 347 357.
Lawson, T. L. and R. Lal. 1980. Response of maize to surface and buried straw mulch on a tropical Alfisol. In: R. Lal (ed.), Soil Tillage and Crop Production. Ibadan, Nigeria: IITA, pp. 63 74.
Lawson, T. L. and E. R. Terry. 1984. Weather and plant disease in Africa. In: D. L. Hawksworth (ed.), Advancing Agricultural Production in Africa. Proceedings of CAB's First Scientific Conference, Arusha, Tanzania, 12 18 February 1984. Slough, UK: CABI, pp. 314 320.
Linnemann, H., J. de Hoogh, M. A. Keyser, and H. D. J. van Heemst. 1979. MOIRA: Model of International Relations in Agriculture. New York: North-Holland.
Loutfi, M. F. 1980. Rural Women Unequal Partners in Development. A Wep study. Geneva: ILO.
Lynam, J. K. and R. W. Herdt. 1988. Sense and sustainability: Sustainability as an objective in international agricultural research. Paper presented at a CIP Rockefeller Conference on Farmers and Food Systems, Lima, Peru, September.
Matlon, P. J. 1987. The West African Semi-arid Tropics. In: J. W. Mellor, C. L. Delgado, and M. J. Blackie (eds.), Accelerating Food Production in Sub-Saharan Africa. Baltimore, Md.: Johns Hopkins University Press.
Maxwell, G. F. 1990. Integrated pest management: Considerations for success in developing countries. In: S. K. Hahn and F. E. Caveness (eds.), Integrated Pest Management for Tropical Root and Tuber Crops. Proc. Workshop on the Global Starus of; and Prospects for, IPM for Root and Tuber Crops in the Tropics. Ibadan, Nigeria: IITA.
Mellor, J. W., C. L. Delgado, and M. J. Blackie (eds.) 1987. Accelerating Food Production in Sub-Saharan Africa. Baltimore, Md.: Johns Hopkins University Press.
Merrill-Sands, D. 1992. Strategic initiatives in roles for FSR/E: Institutional linkages - linkages and impact: Station Research. 12th Annual AFSRE (Association of Farming Systems Research and Extension) meeting, 13-18 September, Michigan State University, East Lansing.
Mulongoy, K. and I. O. Akobundu. 1985. Nitrogen uptake in live mulch systems. In: B. T. Kang and J. van der Heide (eds.), Nitrogen Management in Farming Systems in Humid and Sub-humid Tropics. Harren: Institute of Soil Fertility, pp. 285 290.
Muntemba, S. 1982. Women as food producers and suppliers in the twentieth century: The case of Zambia. Development Dialogue 1-2: 29-50.
Neuenschwander, P. and W. N. O. Hammond. 1987. Biological control of the cassava mealybug, Phenacoccus manihoti, in Africa: A review of field studies. In: S. K. Hahn and F. E. Caveness (eds.), Integrated Pest Management for Tropical Root and Tuber Crops. Ibadan, Nigeria: IITA, pp. 42-57.
Ngambeki, D. S. 1985. Economic evaluation of alley cropping Leucacna with maizemaize and maize-cowpea in southern Nigeria. Agricultural Systems 17(4): 243258.
Norman, D. W. 1982. The Farming Systems Approach to Researck. Farming Systems Research Paper no. 3, Kansas State University, Manhattan, Kansas.
Nwanze, K. F., K. Leuchner, and H. C. Ezumah. 1979. The cassava mealybug: Phenacoccus sp. in the Republic of Zaire. PANS 25(2): 125 130.
Nye, P. H. and D. J. Greenland. 1960. The Soil under Shifting Cultivation. Technical Communication no. 51, Commonwealth Agricultural Bureau, England.
Okafor, J. C. 1980. Edible indigenous woody plants in rural economy of Nigeria forest zone. Forest Ecology and Management 3: 45 55.
Okigbo, B. N. 1977. Effects of mulching with a range of materials on crop yield. In: Mulching and soil management. IITA, Annual Report, Farming Systems Program.
---- 1980. Ahajioku Lecture. Government Printer, Ministry of Information and Culture, Imo State, Nigeria.
---- 1989. Development of Sustainable Agricultural Production Systems in Africa: Roles of the International Agricultural Research Centers and National Research Systems. First Distinguished African Scientist Lecturer Seties, IITA, Ibadan, Nigeria.
Okigbo, B. N. and D. J. Greenland. 1976. Intercropping systems in tropical Africa. In: R. 1. Papendick, P. A. Sanchez, and G. B. Triplett (eds.), Multiple Cropping. Special Publication 27. Madison, Wis.: American Society of Agronomy, pp. 63-101.
Okuneye, B. 1984. Farmers' characteristics and the theory and strategy of agricultural under-development and development. Paper presented at the National Seminar on Innovative Approaches to Development Theory. 23-27 January, Nigerian Institute for Social and Economic Research (NISER), University of Ibadan, Ibadan, Nigeria.
Olasantan, F. O. 1992. Nutrient requirement of cassava (Manihot esculenta Grantz)/ maize (Zea Mays L) intercrop. Ph.D. thesis, University of Ibadan, Nigeria.
Olayide, S. O. and F. S. Idachaba. 1987. Input and output marketing systems: A Nigerian case. In: J. W. Mellor, C. L. Delgado, and M. J. Blackie (eds.), Accelerating Food Production in Sub-Saharan Africa. Baltimore, Md.: Johns Hopkins University Press.
Osiname, O. A., C. Bartlett, N. Mbulu, L. Simba, and K. Land. 1987. Diagnostic survey of cassava-based cropping systems in two ecological zones of gas-Zaire. In: H. C. Ezumah and D. S. O. Osiru (eds.), Cassava Based Cropping Systems Research. Ibadan, Nigeria: IITA.
Osiru, D. S. O. and R. W. Willey. 1972. Studies on mixtures of dwarf sorghum and beans (Phaseous vulgaris) with particular emphasis on plant population. Journal of Agricultural Research, Cambridge 79(3): 531-540.
Pala, A. O. 1976. African Women in Rural Development: Research Priorities. OLC (Overseas Liaison Committee, American Council on Education) Working Paper no. 12, Washington D.C., December.
Pankhurst, D. and S. Jacobs. 1988. Land tenure, gender relations and agricultural production: The case of Zimbabwe's peasantry. In: J. Davison (ed.), Agriculture, Women and Land: The African Experience. Boulder and London: Westview Press.
Papadakis, J. 1966. Crop Ecologic Survey in West Africa, vols. I and II. Rome: FAO.
Paulino, L. A. 1987. The evolving food situation. In: J. W. Mellor, C. L. Delgado, and M. J. Blackie (eds.), Accelerating Food Production in Sub-Saharan Africa. Baltimore, Md.: Johns Hopkins University Press, chap. 23.
Rao, M. R. and R. W. Willey. 1983. Effect of genotype in cereal/cowpea intercropping on the alfisols of semiarid tropics of India. Experimental Agriculture 19: 67 78.
Roberts, P. A. 1988. Rural women's access to labour in West Africa. In: S. B. Stichter and J. L. Parpart (eds.), Patriarchy and Class: African Women in the Home and the Workforce. Boulder, Colo.: Westview Press, pp. 97-144.
Rogers, B. 1980. The Domestication of Women: Discrimination in Developing Societies. London: Kegan Paul.
Ruttan, V. W. 1988. Sustainability is not enough. American Journal of Alternative Agriculture 3: 128 130.
Ter Kuile, C. H. H. 1987. The humid and sub-humid tropics. In: J. W. Mellor, C. L. Delgado, and M. J. Blackie (eds.), Accelerating Food Production in Sub-Saharan Africa. Baltimore, Md.: Johns Hopkins University Press.
Unamma, R. P. A., S. O. Odurukwe, H. E. Okereke, L. S. O. Ene, and O. O. Okoli. 1985. Farming Systems in Nigeria. Agricultural Extension & Research Liaison Services, National Root Crops Research Institute, Umudike, Umuahia, Nigeria.
Vallaeys, G., M. T. Silvestre, M. J. Blackie, and C. L. Delgado. 1987. Development and extension of agricultural production technology. In: J. W. Mellor, C. L. Delgado, and M. J. Blackie (eds.), Accelerating Food Production in Sub-Saharan Africa. Baltimore, Md.: Johns Hopkins University Press.
Wahua, T. A. T. 1983. Nutrient uptake by intercropped maize and cowpeas and a concept of nutrient supplementation index (NSI). Experimental Agriculture 19: 263 275.
Whitlow, R. 1987. A national soil survey for Zimbabwe. Journal of Soil and Water Conservation, July/August.
Willey, R. W. 1979. Intercropping - Its importance and research needs. Part I. Competition and yield advantages. Field Crops Abstracts 32(1): 1 10.
Wit, C. T. de, H. H. Van Laar, and H. van Keulen. 1979. Physiological potential of
crop production. In: J. Sneep and A. J. T. Hendriksen (eds.), Plant Breeding Perspectives. Wageningen, The Netherlands: PUDOC, pp. 47-82.
World Bank. 1989. Sub-Saharan Africa. From Crisis to Sustainable Growth. A Long Term Perspective Study. Washington D.C.: World Bank.
Zok, S. and L. M. Nyochembeng. 1992. Micropropagation of a tropical tuber food crop, Xanthosoma Sagittifolium (L) Schott, in Cameroon. Jay P. Johnson Biotechnology Laboratory, Institute of Agronomic Research, PMB 25 Buea, Cameroon, mimeo.
