Application of Monoculture Yield/Density Relationships to Plant Competition in Binary Additive Series

Binary additive series experiments involving various pairedcombinations of barnyardgrass (BYG), redroot pigweed (RPW),green foxtail (GFT) and rape (RPS) were undertaken in fieldplots in 1980 and 1981. The experimental design used permittedinvestigation of the performance of a one species (the ‘competitor’) over a range of densities, in the presenceof different constant densities of a second species (the ‘indicator’),and as pure stands. Top growth (d.wt after approximately 95d) in monoculture was found to be well-described by de Wit'sspacing formula where M_s is yield, s is space per seed or plant, is maximum(asymptotic) yield at infinite density and ß is spaceper seed or plant at half the maximum yield (i.e. /2). The formulawas also found to fit the observed yields of a competing speciesover the entire range of densities of an indicator. Correlationcoefficients between observed yields and values calculated bymeans of the spacing formula ranged from 0.934 to 0.999, withmost greater than 0.990. The changes in the slopes of a competingspecies' yield curves as indicator density increases reflectthe competitive abilities of the pair of species present. Barnyardgrassand RPS were found to be strong competitors against RPW andGFT, and RPW strongly competed against GFT. The spacing formulacan thus be used to predict the performance of a competing speciesin additive series experiments, and thereby provides anothermeans of investigating crop-weed and other interactions. Echinochloa crusgalli (L.) Beauv., Setaria viridis (L.) Beauv., Amaranthus retroflexus L., Brassica napus L., barnyardgrass, green foxtail, redroot pigweed, rape, competition, binary additive series, yield/density relationships