Maize production in the SSA grew slowly until the early part of the 20th century when it became popular with farmers (De Vries and Toenniessen, 2001). Maize production trends in SSA have fluctuated since the 1960s. Eastern and southern Africa are the predominant maize growing regions, with about 6 million tonnes produced per year until approximately 1985 (FAO, 2000c). West Africa, which produced about 2.5 million tonnes of maize until 1985, saw a 15.4% growth in maize production until 1989, while eastern Africa had no growth in production during this period. Growth in maize production dropped in SSA in the 1990s from 7.3% to 0.5%, resulting in a critical imbalance between maize production and the increasing human population (about 3% per annum) (FAO, 2000c). This period was followed by a phenomenal increase in maize production in SSA, especially in Nigeria, due in part to the availability of improved hybrid maize varieties from IITA. In the 1980s and early 1990s, small-holder maize expanded rapidly at the expense of sorghum and root crops, especially in the more northern drier part of the Guinea Savanna, as a result of diffusion of early-maturing maize varieties (Dixon et al., 2001). Reductions in yield are generally due to environmental stresses such as drought, low soil fertility and the parasitic weed, Striga hermonthica, pests and diseases, and inadequate access to fertlizers.

Sorghum. Sorghum (Sorghum bicolor) is an important staple food and a major source of energy in SSA. It contains three major macronutrients, 74% carbohydrate, 9 to 12% protein and 3% fat (Kochlar, 1986). Following the high cost of wheat and its low production in SSA, sorghum has been used to make nonwheat bread, which is cheap and readily available. Sorghum is used for the production of cake, sausage and biscuits as well as industrial lager beer and locally brewed alcohol. Sorghum is drought-resistant and mainly grown in the semiarid tropics as it is able to cope with drought stress.

Sorghum grain yields in SSA had been low up to the first half of the 20th century, with yields of between 500 to 800 kg ha-1 (Sharma and Nwanze, 1997). The main factors responsible for this low yield have been pests, diseases, weeds (especially Striga hermonthica) and severe drought.

Rice. Rice (Oryzae sativa L.) is an important staple food and is crucial to the economy of many countries in SSA. Rice contains 91% carbohydrate, 7% protein and 0.4% fat (Grist, 1953). The protein lacks sufficient essential amino acids (e.g., methionine, lysine and threonine) and is hence nutritionally inadequate (Chandler, 1979). It is however, a good source of energy for a majority of the populace in SSA as its carbohydrate is easily digestible. It also has enduring palatability and has been used consistently in meals. Rice production in SSA in the pre-colonial era was low as the crop was grown in small areas by resource-poor farmers for subsistence. Rice production increased from the beginning of the 20th century as the crop became not only a subsistence crop, but a source of income for farmers. Rice production in SSA increased slowly in the 1960s from 4 million to over 5 million tonnes in the 1970s, before a sharp rise to over 10 million tonnes in 1990s. This was followed by a fluctuating

increase up to 2000 (WARDA, 2005b). Presently, West Africa is the predominant rice producing region, with the bulk of its production coming from Nigeria. Production levels in Southern Africa are highly influenced by Madagascar, while in Eastern Africa, Tanzania accounted for 80% of rice production. In Madagascar, the development and diffusion of the System of Rice Intensification (SRI) has resulted in very high rice yields up to 17 tonnes ha-1 (Uphoff et al., 2002).

Reduction in yield has been due to environmental stresses such as drought especially in nonirrigated rice, low soil fertility, weeds, insect pests and diseases. Non-utilization of available AKST due to low literacy levels has been one of the contributing factors to low yields recorded before the 1960s. Improved rice varieties are now available in SSA from West African Rice Development Authority (WARDA), following several years of research.

Cowpea. Cowpea (Vigna unguiculata L. Walp) is an important food legume and an essential component of cropping systems in SSA. Cowpea is an important source of nourishment especially for poor people who cannot afford animal protein. Cowpea contributes to soil fertility through its ability to fix nitrogen, which remains in the soil and contributes to subsequent crops. Cowpea haulms contain over 15% protein and constitute a valuable source of fodder for livestock (Dike, 2005). Cowpea production in SSA before the 20th century and in the first half of the century had been low mainly due to the use of local cowpea varieties and traditional farming systems. Increased production of cowpea has resulted in an increase in the quality of food available in SSA. This has ensured healthier livelihoods through the reduction of diseases such as kwashiorkor and protein energy malnutrition. As a legume with high protein content, it has proved an essential dietary component for the mitigation of diseases such as diabetes.

Mean cowpea grain yields in traditional intercropping systems range from 0 to 132 kg ha-1, depending on the fertility level of the fields (Van Ek et al., 1997), compared with a sole cowpea yield potential of 1,500 to 3,000 kg ha-1 under optimum management (Muleba and Ezumah, 1985).The use of improved cowpea varieties, an increase in harvested areas and improved crop cultural practices have contributed to increases in production. Drought stress and poor soil fertility have been important factors resulting in reduced cowpea yields.

Horticultural crops. Many horticultural crops are grown in SSA. These include vegetable crops such as tomatoes, onions, peppers, garlic, eggplants, lettuce, carrots, watermelons, melons, cabbage, spinach, pineapples, apples, bananas, plantains and potatoes, and several fruit trees such as mangoes, guava, cashew, oranges and other citrus crops. These crops are rich in vitamins A, C and E and contribute to the quality of local diets and nourishment available for the maintenance of good health in the increasing population in SSA. Horticultural crops also have export value and contribute immensely to the export earnings of several countries in SSA. Tanzania is the largest exporter of horticultural crops in East Africa with cashew nuts alone accounting for 70% of horticultural export. Crop yield