GENOTYPIC REQUIREMENT FOR INTERCROPPING

An important aspect of an intercropping system is the extent of competition between the crops. Where this is large, there is likely to be a significant genotypexcropping systems interaction. Even when the correlation between sole-crop and intercrop yields is positive, there may be significant departures from the trend which can be exploited by plant breeding. The traits required for intercropping are those which enhance the complementary effect between species. The setting of objectives must take into account not only the influence of a particular trait on the target crop's yield, but its potential effect on another crop. A useful strategy for areas with unreliable rainfall distribution is to intercrop a relatively short-duration crop with a long-season indeterminate crop, for which the periods of maximum demand on water resources are different. Photosensitivity provides a mechanism for timing precisely the flowering of particular cultivars. The plant types of the intercropped species should be selected to complement each other. Cultivars of species which are compatible in this way can be planted at higher density, and this is a major factor contributing to the ability of intercrops to yield more than sole crops. Plant breeding is costly. An intercropping system targeted for breeding must clearly be of major significance, cover a large area and be sufficiently unique to justify a separate programme. A cereal/legume intercrop in excess of 0. 5 million hectares is estimated to be sufficiently large to justify a full-scale crossing and selection programme. For adaptive variety trials bivariate analysis is the preferred approach, in order to remove background correlated effects between the two crop yields in an intercrop. An alternative is to select on the basis of income. Farmers, however, may use a wider range of criteria to select cultivars. The best way of accounting for these is to involve farmers in the selection process. Early generation selection in a breeding programme should first focus on the highly heritable traits identified as contributing to intercrop efficiency. Early generation yield evaluation of genotypes of beans intercropped with maize was found to be feasible and effective. Selection under farmers' own management should be introduced at as early a stage as possible, once a set of promising lines has been identified. Breeding for intercropping will be most effectively carried out in the context of regional collaboration or international programmes, focusing on improving and extending the use of cropping systems which are already widely practiced.