Competition for light and nitrogen during a regrowth cycle in a tropical forage mixture

Abstract

Abstract A dynamic analysis of light and nitrogen sharing in a grass/legume mixture during a regrowth cycle in a humid tropical climate is presented. Pure stands and mixture plots of two stoloniferous species, Arachis pintoi (a C3 legume species) and Digitaria decumbens (a C4 grass species), were established on ferralitic soil in randomised sub-plots with four replicates. Each crop was fertilised at two different nitrogen levels: 150 (N150) and 0 (N0) kg of nitrogen per hectare. Growth was analysed in terms of radiation-use efficiency (RUE) of photosynthetically active radiation (PAR) absorbed by crops. Daily absorption efficiencies of the mixture's components were estimated by a radiative-exchange model. The results demonstrated that presence of the legume reduced light interception by the grass by 13 and 28% under the N150 and N0 conditions respectively. On the other hand, competition by D. decumbens for light was very strong at both nitrogen levels since it reduced interception of incident PAR by A. pintoi by 69% (N0) and 91% (N150) in the growth cycle. Intercropping did not result in changes in the radiation-use efficiency of either species except in N150 mixed crop where A. pintoi virtually disappeared due to zero radiation interception at the end of the cycle. Uptake of nitrogen by the grass was not affected by presence of the legume. Despite different production potentials of C3 and C4 species, under N0 mixed-crop conditions, behaviour was neutral. Consequently, the mixture could be maintained at equilibrium if sward production is not intensified by nitrogen fertilisation.