Publication Details

Lemerle, D., Cousens, R. D., Gill, G. S., Peltzer, S. J., Moerkerk, M., Murphy, C. E., Collins, D. & Cullis, B. R. (2004). Reliability of higher seeding rates of wheat for increased competitiveness with weeds in low rainfall environments. Journal of Agricultural Science, 142 (4), 395-409.


Increasing crop competitiveness using higher seeding rates is a possible technique for weed management in low input and organic farming systems or when herbicide resistance develops in weeds. A range of wheat seeding rates were sown and resulted in crop densities between 50–400 plants/m2 (current recommendations are 100–150 plants/m2) in the presence and absence of annual ryegrass (Lolium rigidum Gaud.) in three wheat cultivars at nine experiments in southern Australia. Wheat densities of at least 200 plants/m2 were required to suppress L. rigidum and to a lesser extent increase crop yield across a wide range of environments (seasonal rainfall between 200–420 mm) and weed densities (50–450 L. rigidum plants/m2). Doubling crop density of all cultivars from 100 to 200 plants/m2 halved L. rigidum dry weight (averaged over all experiments) from 100 g/m2 to about 50 g/m2. Higher crop densities gave diminishing marginal reductions in weed biomass, while cultivar differences in weed suppression were small. Grain yields ranged from 0·5 t/ha to over 5 t/ha depending on site and season. Maximum yields in the weed-free plots (averaged over environments and cultivars) were at 200 crop plants/m2, and yield declined only slightly by 4–5% at densities up to 425 plants/m2. In the weedy plots grain yield continued to increase up to the highest density but at a slower rate. The percentage yield loss from weed competition was of a smaller magnitude than the suppression of L. rigidum by wheat. For example, 100 wheat plants/m2 led to an average 23% yield loss compared with 17% at 200 plants/m2, and the probability of reduced crop grain size and increased proportion of small seeds was negligible at these densities. Cultivar differences in yield loss from weed competition were small compared with differences due to crop density. Adoption of higher wheat seed rates as part of integrated weed management is now strongly promoted to farmers.



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