A simple competitive ratio (CR) is proposed as a measure of intercrop competition, to indicate the number of times by which one component crop is more competitive than the other. Intercropping data show that this CR term could be useful in (i) comparing the competitive ability of different crops, (ii) measuring competitive changes within a given combination, (iii) identifying which plant characters are associated with competitive ability, and (iv) determining what competitive balance between components is most likely to give maximum yield advantages.

S U M M A R Y Data from 94 experiments on sorghum/pigeonpea intercropping were examined for evidence that the stability of yield is greater with intercropping than sole cropping. Stability of the major component (sorghum) was examined by calculating the distribution of yields; stability of the overall intercropping system was examined by calculating coefficients of variation, by computing regressions of yield against an environmental index, and by estimating the probability of monetary returns falling below given 'disaster' levels. All these approaches have some merit; taking the last as an example, it was found that for a particular 'disaster' level quoted, sole pigeonpea would fail one year in five, sole sorghum one year in eight, but intercropping only one year in thirty-six. Intercropping gave yield advantages under a wide range of environmental conditions and there was no significant evidence that advantages were greater under stress. This is discussed in relation to possible mechanisms contributing to greater yield stability.

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Nodulation and N,-fixation of groundnut were investigated in sole and intercropping systems.Intercropping with pearl millet, maize, or sorghum reduced nodulation and N 2 -fixation. This effect is ascribed to the shading of groundnut by the cereal component and the consequent decrease in photosynthesis of the legume canopy.

This paper tries to illustrate how beneficial interactions between crops can be exploited to increase the overall output of a cropping system. Growth studies with sorghum/pigeonpea and millet/groundnut are described to show how intercropping systems can achieve much larger yields than sole crops by using environmental resources more fully over time or more efficiently in space. Data from moisture stress studies are presented to illustrate that these advantages of intercropping can be even greater under stress conditions.