Associated growth of wheat and annual ryegrass. 2. Effect of varying the time of ryegrass germination in stands of wheat

Abstract

In a field experiment, wheat cv. Gamenya and annual ryegrass cv. Wimmera were sown in four proportions, 80W : 20R, 60W : 40R, 40W : 60R, 20W : 80R, at a total density of 300 plants m-2 and in which a high level of nitrogen fertilizer application was maintained. Ryegrass was sown 17 days before, simultaneously with, and 17 days after wheat. At the simultaneous sowing there were also pure cultures of wheat and ryegrass; pure culture data for ryegrass corresponding to all the germination treatments were obtained in a supplementary experiment. All treatments were harvested on five occasions. Observations were made on tiller numbers, yield of dry matter, seed numbers and the yield components of wheat. There were marked effects of early and simultaneous sowing of ryegrass and of proportions; the results also showed interactions between the later sowings and proportions. In all treatments wheat tiller numbers rose to a maximum and declined to about half after about 100 days. A similar rise and fall was observed in the early and simultaneous germination of ryegrass, but not in the late one. In ryegrass, tiller numbers were much greater than in wheat, and the rate of decline was not as marked. In the late germination treatment ryegrass development and growth were seriously depressed. With respect to wheat, seed yield and ear numbers per unit area in the early and simultaneous sowings were less, while ears and grains per plant were greater in the presence of ryegrass than in pure culture. In ryegrass, seed production was highest in the early and least in the late treatments and was depressed as the proportion of wheat was increased; even in the treatment with the lowest proportion of ryegrass, seed numbers of ryegrass remained high. Relative reproductive rates showed a consistent pattern with time of sowing and with proportion, and the results suggest that control of ryegrass seed numbers cannot be achieved by manipulating planting treatments but requires positive intervention. Relative crowding coefficients (k) calculated from the data suggest that wheat was the stronger competitor. The results are discussed in terms of the competitive effects between species and the likelihood that wheat has the greater competitive ability. As in the previous study, it was noted that effects of competition were discernible at 28 days.