Comparing biological productivity in cropping systems: a competition approach

1. Intercropping, the practice of growing two or more crops in close association, is attracting increasing interest in developed countries, primarily due to claims that it can provide increased yields in an environmentally sustainable manner. 2. The land equivalent ratio (LER) is frequently used to attest the claim that intercropping produces a greater total biological productivity per unit area of land than monocropping. However, similar to other indices, the value of the LER is dependent on the relative densities of the two crops and fails to provide an indication of the optimal crop density combination required to produce maximum biological productivity. We used an alternative methodology to predict the optimum density for planting two crops in a mixed stand. 3. Maize and beans were grown both in monoculture and in intercropped stands at a range of densities. Monocrop stands of both maize and bean displayed an asymptotic yield–density relationship. When a second crop was introduced into the stand there was no effect on the yield of maize, but there was a large reduction in the yield of bean. 4. The LER did not demonstrate an unequivocal advantage to intercropping. Less than a third of the LER values, over the range of densities considered, were greater than one, although none was significantly different to unity. The highest values were obtained from those plots sown with a high density of maize. 5. An alternative method of analysing the biological productivity of an intercrop system is proposed. This approach requires a combination of (i) an additive experimental design, (ii) the use of a regression model to measure the competitive effect of two species, and (iii) the presentation of the net competitive effect on individual and total yields using a response surface. The reciprocal model was used to dissociate and quantify intra- and interspecific competition, estimate a competitive equivalence coefficient and predict the individual yields of the full complement of density combinations included in the maize and bean experiment. 6. The model explained up to 90% of the variation in the observed yields of maize and bean in an intercrop. An inspection of the response surface for the LER indicated that the minimum density combination required to produce the maximum yield advantage within the density range considered comprised maize being planted at a density of 11 plants m−2 and bean being planted at a density of 39 plants m−2. Although this combination would provide the maximum biological productivity, it is considered unlikely to provide the maximum gross economic margin. 7. The above methodology may provide a useful tool for managing competition, either between two or more crops in an intercrop, or within a crop–weed interaction