COMPETITION FOR LIGHT AND NITROGEN DURING A REGROWTH CYCLE IN A TROPICAL FORAGE MIXTURE

A dynamic analysis of light and nitrogen sharing in a grass/legume mix ture during a regrowth cycle in a humid tropical climate is presented. Pure stands and mixture plots of two stoloniferous species, Arachis p intoi (a C3 legume species) and Digitaria decumbens (a C4 grass specie s), were established on ferralitic soil in randomised sub-plots with f our replicates. Each crop was fertilised at two different nitrogen lev els: 150 (N150) and 0 (N0) kg of nitrogen per hectare. Growth was anal ysed in terms of radiation-use efficiency (RUE) of photosynthetically active radiation (PAR) absorbed by crops. Daily absorption efficiencie s of the mixture's components were estimated by a radiative-exchange m odel. The results demonstrated that presence of the legume reduced lig ht interception by the grass by 13 and 28% under the N150 and NO condi tions respectively. On the other hand, competition by D. decumbens for light was very strong at both nitrogen levels since it reduced interc eption of incident PAR by A. pintoi by 69% (NO) and 91% (N150) in the growth cycle. Intercropping did not result in changes in die radiation -use efficiency of either species except in N150 mixed crop where A. p intoi 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 NO mixed-crop conditions, behaviour was neutral. Consequently, the mixture could be maintained at equilibrium if sward production is not intensified by nitrogen fertilisation.