Suppression of weeds by spring wheat Triticum aestivum increases with cropdensity and spatial uniformity

1. Recent advances in our understanding of the advantage of initial size in competition among individual plants (size-asymmetric competition) suggest that the potential for many crops to suppress weeds is much greater than ge nerally appreciated. We hypothesize that this potential can be realized if. (i) the crop density is increased significantly and (ii) the crop is regul arly (uniformly) distributed in two-dimensional space rather than sown in t raditional rows. 2. We tested these hypotheses by sowing four varieties of spring wheat Trit icum aestivum at three densities (200, 400 and 600 m(-2)) and in two spatia l patterns (normal rows and a uniform grid pattern) in the presence of high weed pressure. 3. There were strong and significant effects of both crop density and spati al distribution on weed growth. Weed biomass decreased with crop density an d was 30% lower in the grid pattern. 4. There was a negative linear relationship between above-ground weed bioma ss in early July and crop yield at harvest, so weed suppression translated directly into yield. The treatment with high crop density and the grid sowi ng pattern contained 60% less weed biomass and produced 60% higher yield th an the treatment closest to normal sowing practices (crops sown in rows at 400 m(-2)). 5. The results were similar when the experiment was repeated in the followi ng year, even though weed abundance was lower and the weed community was ve ry different. There was 30% less weed biomass and 9% higher yield when the crop was sown in a grid pattern. 6. While weed biomass decreased monotonically with density for all varietie s, a significant variety-density interaction suggested that the attributes resulting in good weed suppression at high crop density may not be the same as those most advantageous at low crop density. 7. A more crowded, uniform, distribution of some crops could contribute to a strategy to reduce the use of herbicides and energy-intensive forms of we ed control.