THE ANALYSIS OF MULTIPLE CROPPING SYSTEMS IN AFRICA

Farmers adopt multiple cropping systems for a variety of pragmatic reasons, and Ruthenberg (1980) distinguishes between mixed cropping and phased planting.

Mixed cropping is adopted:

• to increase overall production per unit area

• to reduce the incidence of pests and diseases

• to enable planting to take account of soil variations

• to exploit the different mature heights of various crops

• to ensure a continued and varied supply of food

• to provide a soil cover against weeds and erosion

• to even out the demand for labour during the year

Phased planting is adopted:

• to even out the demand for labour during the year

• to reduce land preparation costs

• to minimise risk, particularly from climatic uncertainities

• to provide phased harvests

• to provide ground cover.

The disadvantages of mixed cropping systems indicated below are more apparent to those who wish to change them:

• difficulties of mechanisation

• difficulties in applying inputs, e.g. fertilizers

• more complex experimentation (Okigbo & Greenland, 1976).

Analyses of traditional systems generally show overall increases in crop yields. For dry zone farms incorporating A. albida trees (up to about 40 trees/ha) Charreau & Vidal (1965) record striking effects on the yield of millet, for example:

Table 3 Effects of A. albida. on millet yield in Senegal

Note that not only was the quantity of grain increased; the quality was also improved. Great variations between years were experienced.

Soil analyses in the same cropping zones showed equally interesting results. Table

4. Soil analyses near A. albida trees in Senegal  

The usual measure of improvement in yield resulting from multiple cropping is the Land Equivalent Ratio (LER). This is the relative land area under sole crops required to produce the same yields from a unit area of intercropping.

Some recent examples quoted from Beets (1982) show very substantial improvements in yield from field studies; such improvements are, of course, very evident to the farmer.

Table 5 Examples of improved yields obtained through intercropping, Land Equivalent Ratios (LER) Beets (1982) & others

The benefits of mixed cropping are not only in yields, but there is also mounting evidence of the positive effects on soil conservation, and thus on the sustainability of the systems; (see, for instance, Table 6).

Table 6 Annual soil loss in tonnes/ha under monocrops at Ibadan, Nigeria Annual and mixed cropping (after Aina et al., 1076)

The socio-economic basis on which the African farmer makes his decisions has been studied by a number of writers. The small farmer is not particularly impressed by an LER obtained from on-station experiments, however impressive in percentage terms these may be (see Table 5). The actual increase in harvestable yield may, in fact, be very small on a small field or farm. Farmers are more likely to adopt multiple cropping for reasons other than the LER, for instance a more efficient use of labour per unit of output, particularly in intensity of cultivation and weeding. This is despite the fact that the labour input per unit area is often increased.

An equally important reason for traditional intercropping is erratic rainfall. Steiner (1982) records that two-thirds of crop failures in the southern Guinea zone can be attributed to this cause, and the 'contingency mixing of maize and sorghum in Haute Volta is a good example of a response to this risk.

In some societies there is a traditional division of responsibility for crops between the sexes - women in parts of Cameroun, for instance, plant 'their' crops in their husbands fields. The result inevitably is a complex multicropping system. In many areas near to a powerful market the opposite - the adoption of low-labour monocropping - is the farmers' response.

The economic evaluation of taungya has been studied in both West and East Africa and, from the point of view of the national government generally shows substantial economic advantages for the establishment of plantations. Ball (1977) gives realistically estimated internal rates of return (IRR) for various methods of raising plantations of Tectona grandis (teak) and Gmelina arborea in southern Nigeria.

Table 7. Costs and IRR of plantations in Nigeria 1975-76. Agricultural crops, maize, yams, cassava, vegetables

Note the effect of time on the profitability of Gmelina, a much less valuable crop than teak.

In Congo the raising of Terminalia superba with bananas is an extremely successful taungya system in which the now maturing trees have been underplanted with cocoa (Koyo, 1981). Here, however, the principles of agroforestry mixtures were insufficiently applied, and, although the (unthinned) plantation trees continued to grow reasonably well, the yields of cocoa fell over the years 1970-73, giving 49, 67, 31 and 16 kg/ha respectively.

Footnote_______________

^1/ Directly employed labour. Trees and agricultural crops sold by Forest Department.

^2/ Farmers own and sell agricultural crops and clear land free.

^3/ All labour employed direct. No agricultural, crops.

^4/ Teak rotation 60 years, Gmelina 15 years.