Suppression of weeds by spring wheat Triticum aestivum increases with crop density 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 generally appreciated. We hypothesize that this potential can be realized if. (i) the crop density is increased significantly and (ii) the crop is regularly (uniformly) distributed in two-dimensional space rather than sown in traditional rows. 2. We tested these hypotheses by sowing four varieties of spring wheat Triticum aestivum at three densities (200, 400 and 600 m(-2)) and in two spatial 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 spatial distribution on weed growth. Weed biomass decreased with crop density and was 30% lower in the grid pattern. 4. There was a negative linear relationship between above-ground weed biomass in early July and crop yield at harvest, so weed suppression translated directly into yield. The treatment with high crop density and the grid sowing pattern contained 60% less weed biomass and produced 60% higher yield than 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 following year, even though weed abundance was lower and the weed community was very 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 varieties, 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 weed control.