Recent studies have shown major advantages of increased crop density and spatial uniformity for competition of wheat with weeds. Field experiments were performed over 3 yr to determine whether the effects of crop density and sowing pattern on weed suppression are influenced by nitrogen fertilization. The independent variables were crop sowing pattern (normal rows and a highly uniform pattern), seeding density (204, 449, and 721 seed m⁻²) and nitrogen fertilization (0 and 80 kg nitrogen ha⁻¹) of spring wheat, grown under high weed pressure. Increased crop density had strong and consistent negative effects on weed biomass and positive effects on crop biomass and yield. At the highest crop density, weed biomass was less than half that at the lowest density. Weed biomass was generally lower, and yield higher, in the uniform pattern, except in one case in which a combination of factors gave one weed species an early size advantage over the crop. When weeds were controlled with herbicide, no effects of crop density or spatial uniformity on crop biomass or yield were observed. Nitrogen fertilization increased weed biomass in 2 of 3 yr, and it also increased crop biomass in 2 of 3 yr, but there was little evidence that the relative effects of crop density and spatial pattern on weed suppression were influenced by nitrogen fertilization. In the presence of weeds, the highest yields were obtained with high crop density, high spatial uniformity and nitrogen fertilization. The results indicate that increased weed suppression through increased crop density and spatial uniformity will occur over a wide range of nitrogen levels.

Nomenclature: Spring wheat, Triticum aestivum L. ‘Leguan’.