WILD OAT (AVENA FATUA L.) COMPETITION WITH SPRING WHEAT (TRITICUM AESTIVUM L.) (INTERFERENCE, WEED, NITROGEN

Field experiments were conducted to measure the grain yield of wheat (Triticum aestivm L. 'ANZA') at various wild oat (Avena fatua L.) and wheat plant densities. Wheat yield declined as wild oat plant density increased. Wheat yield increased in wild oat-infested plots as wheat plant density increased. Regression models were developed to describe the combined effect of wheat and wild oat plant densities on wheat grain yield. Wheat yields were best described by nonlinear regression models using the relative density of wild oat in the weed-crop stand as the dependent variable.;Wheat grain yield in wild oat-infested plots generally declined with nitrogen fertilization while the density of wild oat panicles increased. Apparently, in competition with wheat, wild oat was better able to utilize the added nitrogen and thus gained a competitive advantage over the wheat. Under the conditions of these experiments, nitrogen fertilization resulted in positive wheat yield response only when the wild oat plant density was below 1.6 percent of the total plant density.;A method was developed to calculate the relative competitive ability of wild oat in spring wheat from crop yield regression equations. The method was used to quantify shifts in relative competitive ability of wild oat with changes in plant density, plant species proportion and soil nitrogen availability.;The relative competitive ability of a weed ((alpha)) was defined as the effect of weed density on per plant crop yield compared to the effect of crop density on per plant crop yield. The relative competitive ability of wild oat ((alpha)) declined slightly as total plant density increased. As the proportion (frequency) of wild oat increased, the competitiveness of wild oat declined, apparently due to increased intraspecific competition between wild oats. Nitrogen fertilization increased the competitive ability of wild oat at all plant densities tested. At nitrogen fertilizer levels above 80 kg/ha, intraspecific competition between wild oat appeared less critical and the relative competitive ability of wild oat increased with increased wild oat density.