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close this bookSustaining the Future. Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 p.)
close this folderPart 3: Environment and resource management
close this folderAgricultural development in the age of sustainability: Crop production
View the document(introduction...)
View the documentIntroduction
View the documentThe ecological zones of Sub-Saharan Africa
View the documentGeneral crop production constraints and potentials for overcoming them
View the documentTechnologies with potential for sustainable resource management
View the documentWomen's underexploited potential
View the documentSuggested approaches to sustainable production
View the documentSummary
View the documentConclusions
View the documentAcknowledgements
View the documentReferences

Technologies with potential for sustainable resource management

Good technologies with missing links

In Sub-Saharan Africa, considerable efforts have been expended on the development of improved cereals, root and tuber crops, and food and fodder legumes by plant breeders, and also on the characterization and identification of the limitations of the soils. Pest control measures have also received attention, particularly from international (IITA, IRRI, ICRISAT, WARDA, ILCA, ICP),1 national, and other research centres and universities in Africa. Agronomists and soil scientists have conducted a lot of research on responses to fertilizer application of various crops in different ecological settings, and breeders and disease and pest control specialists have documented results based on chemical, host plant resistance, biological, and chemical control measures.

Each of these results, introduced into a farmer's system, provides some relief to problems. The ephemeral nature of some of the relief is realized when the breeders' variety yields less than expected in the intensive multistorey crop association system of small-scale farmers (duo and Ezumah 1992), and when the expected response to fertilizer is not realized, either because the increased crop pressure requires higher applications (Wahua 1983; Olasantan 1992), or because the soil physical conditions have deteriorated so much that the effectiveness of fertilizer is reduced in intensive systems (Lal and Greenland 1978). Similarly, the undesirable long-term effects of pest and disease control by a non-integrated approach have been documented (Maxwell 1990). A holistic approach to research and extension, which also incorporates the concepts of integrated pest management (IPM), could reduce the dangers of unsustainable crop production in SubSaharan Africa.

Technical innovation in agriculture is generally not designed so as to exploit the complementary and synergistic effects of the important results for sustained crop production enumerated earlier. Such complementarities are achievable when technologies are developed from current farmers' knowledge base, using multidisciplinary experiences (Norman 1982; Hildebrand 1990). The central thesis is that resourcepoor farmers do not adopt technologies that require costly inputs of labour, cash, and materials or technologies for which inputs are not readily available. These technologies therefore do not fit in the farmers' production environment and frequently break the linkages that enhance resource conservation.

The International Institute of Tropical Agriculture, for example, developed an early maturing erect cowpea that, together with other improved varieties, required frequent applications of insecticide. Among the early varieties were TVX3236 and IT82E-60. As long as chemicals were subsidized when the Nigerian currency, the naira, was relatively strong, some farmers, particularly those on larger farms, grew these cowpea varieties. The majority of the small-scale farmers did not adopt the early, erect types because (a) they required insecticides that were not available or that they could not afford and (b) the vegetation required as animal feed was too scanty. Thus the improved variety that matured early enough in the low-rainfall zones did not satisfy the conditions for its adoption (Carr 1989). With respect to fertilizers, recommendations are available in virtually every country in Sub-Saharan Africa. The limited use of fertilizers, even when prices are subsidized, is attributed to poor distribution systems (Harrison 1987). Although many farmers in south-eastern Nigeria (Unamma et al. 1985) and in Zaire (Osiname et al. 1987) are aware of the importance of fertilizers and herbicides, most are not making use of them because (i) they may not be available either at all or when required, (ii) they are too costly, or (iii) they require equipment that the farmers do not own, e.g. knapsack sprayers. A common feature of the technologies cited - which also include tractorized tillage, liming, short-stalked, high-yielding sorghum for farmers who

may require stalks for fencing or for fuelwood, and zero tillage unaccompanied by a weed control package for reduced tillage systems - is that they do not fit into farmers' production environments because some components that would facilitate their usefulness in existing systems are missing. Adoption of the early cowpea and of dwarf sorghum might, because of high yields, lead to a destruction of vegetation from other environments for animal feed and for fuelwood. It is also noted that the modern practices of conventional agriculture, which comprise land clearing and preparation (tillage systems), fertilization, weed control, and harvesting, have elements of environmental degradation.

Mimicking natural ecosystems

Almost all the sustainable systems currently available in Sub-Saharan Africa mimic natural ecosystems. These systems comprise traditional shifting cultivation, which is sustainable at low population pressures, well-managed multiple-cropping systems (which include compound land systems), the alley cropping system, and the fadama or inland valley systems. These systems may have some or all of the following attributes: extending the duration of growth of the plant community, increasing light-capturing potential through multi-layer interception over a longer period, and recycling nutrients from deep layers. The systems also integrate many groups of plant species - ephemerals, annuals, and perennials - in the same land area. By mimicking nature, microclimates suitable for the growth of many species of plants, and that therefore enhance diversity, are created (Okigbo and Greenland 1976; Juo and Ezumah 1992). Multiple-cropping (i.e. intercropping and rotation) leads to more efficient resource use and this is reflected in yield advantages (Osiru and Willey 1972; Okigbo and Greenland 1976; Willey 1979; Ezumah and Lawson 1984, 1990). Associations that exhibit yield advantage may be long-duration plants intercropped with short-duration plants (Okigbo and Greenland 1976; H. Ezumah 1990) or combinations of short-duration crops belonging to the same family, e.g. sorghum and millet (Willey 1979), or of different species, e.g. sorghum and pigeon pea (Rao and Willey 1983) or maize and cowpea (Ezumah and Ikeorgu 1993). Other advantages of multiple-cropping include improved physical and chemical soil conditions for growth (Lal 1976), reduced soil temperature (Lal 1976; Ikeorgu and Ezumah 1991), reduced soil surface evaporation, and increased water content (Lal 1976). Increased biological activity in the soil (e.g. earthworm activity) in intercropping compared with monocrop rotations (Hullugale and Ezumah 1991) and a reduction in surface runoff and soil loss and therefore a reduction of soil degradation have also been reported for intercropped situations (Aina et al. 1977). In Ouagadougou, Hullugale (1989) showed that undersowing Stylosanthes with maize reduced soil temperature and increased soil moisture, which was significantly improved by conserving moisture through the erection of cross ridges or tying of ridges. Ikeorgu and Ezumah's (1991) results also showed reduced soil temperature in cassava intercropped in four complex mixtures with maize, okra, and egusi melon compared with sole cassava or cassava + maize intercrops. These results highlight the need to focus research on farmers' current systems and to improve on them. They also show the importance of conserving plants and animals in the wild, because their usefulness to humans, apart from the broad concept of ecological balance, is unknown.

Surface mulching

Some of the advantages of multiple cropping are also obtained by surface mulching of the soil with dead material (Lal 1976). Okigbo (1977) studied a wide range of mulching materials including gravel, sawdust from wood, translucent white and black plastics, as well as foliage and twigs from different plant sources including leguminous and non-leguminous plants. The short-term advantages of mulching on an alfisol in Nigeria appear to relate more to improvement of the soil microclimate for plant growth than to chemical properties, because higher crop yields were obtained from the plastic mulches than from the foliages and the twigs. These effects do not, however, negate the long-term benefits of mulching (Lal 1989). A major difficulty of mulching is the procurement of materials. Lawson and Lal (1980) estimated a threshold of 4-6 tons/ha for effective mulching on an alfisol in Nigeria. This quantity of mulch is too much for a lowresource farmer to carry. Akobundu (1980) reported higher maize yield without N fertilization over a five-year period of continuous cropping in association with living legume plants (Psophocarpus palustris and Centrosema pubescens). Little response to N fertilizer was observed in the legume-associated plots compared with the control, which responded to over 60 kg N fertilization per hectare. For greater benefits, a well-established legume plot is required (Mulongoy and Akobundu 1985). Cassava intercropped with maize generates enough mulch to sustain yields. The IITA (1985) reported stable yields of cassava and maize over four years on an alfisol in southern Nigeria. Longer-duration maize (over 120 days to maturity) with high dry matter gave higher yields than shorter-duration maize (less than 100 days to maturity). The short-duration maize allowed in more light to the associated cassava in their intercrop system.

Alley cropping

The most recent innovation in mimicking the natural ecosystem is the alley cropping system (Kang and Wilson 1987). In alley cropping, the multistorey association of the compound land setting is rearranged so that trees occupy adjacent hedges (hedgerows), about 4m apart. Crops are then grown in the alleys between the hedges. Trees are chosen for certain characteristics such as deep rooting (to recycle nutrients), ability to coppice and to produce high biomass (which is pruned for mulching and nutrient release), and, sometimes, rhizobia N-fixing ability. Legume trees in the hedges contribute N in excess of 40 kg/ha to associated crops (Kang 1988). Even non-legumes recycle deeply located nutrients at about 13-19 kg/ha. Although the alley system contributes to moisture conservation, organic residue, and structural and chemical improvements to the soil, a reduction in the yields of associated crops owing to reduced light because of the tree canopy has been observed (Lawson and Kang 1990). Considering its soil improvement features and the stability of alley cropping over a period of years (Kang and Wilson 1987; IITA 1989: fig. 2), alley cropping is a sustainable system that needs refining. Unlike compound farming, it is amenable to large-scale methods. The labour requirements for pruning, which often coincides with other important farm activities (e.g. weeding and harvesting of maize and melon), are a serious setback (Ngambeki 1985). Alley species suitable for acid soils are still being sought. Figure 11.2a shows a rapid decline of maize yields in a continuous cropping system, even if fertilizer is applied. Alley cropping can stabilize maize yields at levels that vary with inputs, e.g. fertilizer (fig. 11.2b). In the latter case, fertilizer augments biologically generated nutrients to sustain maize yields.

Inland valleys

Fig. 11.2 Muize production at IITA, Ibadan. (a) Relationship between length of continuous cultivation and maize yields. (b) Sustainability of maize production in alley cropping systems (Source: Kang and Wilson 1987)

Inland valleys or fadamas, though small individually, total tens of millions of hectares in West and Central Africa. International Institute of Tropical Agriculture estimates for West Africa alone gave about 14 million ha (IITA 1980). Inland valleys are well watered and have enormous potential for producing food, especially rice. In China, inland valleys have been cropped continuously for centuries (duo and Lowe 1986). The neglect of Sub-Saharan Africa's inland valleys, according to the International Institute of Tropical Agriculture (IITA 1990), is attributed to lack of knowledge about their management, which leads to their infection by vectors of many harmful diseases, such as Schistosomiasis, river blindness, malaria, and guinea worms. In addition to water availability, the inland valleys are sustainable and give higher rice yields because they are relatively high in fertility as a result of inflows of nutrients from the uplands. Increased fertility of the inland valleys of Sub-Saharan Africa can be attained by rotating rice with legumes tolerant of water logging, some of which are Crotolaria spp., soybean (Glycine max), and Sesbania spp. Thus, exploitation of the potential for sustainable plant production in the inland valleys of Sub-Saharan Africa requires more research.

Disease and pest control

Some known crop management methods to control diseases and pests include chemical and mechanical measures. These may require high capital and labour inputs. Host plant resistance and biological control measures, though requiring high initial investment at institutional levels, are sustainable and indirectly affordable by the low-resource farmers of Sub-Saharan Africa. Recent examples include the control of cassava bacterial blight (Xanthomonas manihotis) by resistance breeding (Hahn et al. 1979) and the effective reduction of the cassava mealybug pest (Phenacoccus manihoti Mat-Fer) by biological control (Neuenschwander and Hammond 1987). A parasitoid, Epidinocarsis lopezi, introduced from Latin America and released at strategic sites has contributed to the reduction of the cassava mealybug epidemic in Africa.

Many examples of disease and pest resistance through breeding have been documented for various crops in Sub-Saharan Africa. Claims that diseases and pests (except for weeds, for which it has been demonstrated) are controlled by intercropping need further research because reports have been inconsistent (IITA 1978; Francis 1989).