Associated growth of wheat and annual ryegrass. 1. Effect of varying total density and proportion in mixtures of wheat and annual ryegrass

A field study was undertaken in which wheat (T. aestivum L.) cv. Gamenya and annual ryegrass (Lolium rigidum Gaud.) cv. Wimmera were grown at total densities of 33, 100,300 and 900 plants m-2 in pure cultures and in mixtures in the proportions 33W : 67R, 50W : 50R, 67W : 33R. In densities of 100 and 900 plants m-2, five sequential harvests were made for total dry matter of each species in each pure culture and proportion. Interactions between proportion and density at the first and some subsequent harvests were noted, while at others these treatments behaved independently. At maturity, all plots were sampled for dry matter of each species, grain yield in wheat and seed production in ryegrass. With respect to dry matter and seed numbers, there was no interaction between density and proportion, but a main effect of density was noted. Seed numbers in ryegrass were between 31,000 and 45,000 m-2, and increasing the proportion of wheat had little effect on ryegrass seed production. An examination of the yield components of wheat (ears per plant, spikelets per ear and grains per spikelet) showed that intraspecific competition replaced interspecific competition as density rose. Although in pure cultures the two species had a similar growth pattern, the results showed that the effects of the species on one another were not identical and at final harvest, relative yield totals (RYT) > 1.0 were recorded in some treatments. The results are discussed in terms of inter- and intraspecific competition. From the practical point of view, this study shows that increasing either the density or proportion of wheat does not reduce the seed production of ryegrass to any great extent, and therefore its potential as a weed in wheat crops.