Wild oat (Avena ludoviciana) is one of the worst weeds in wheat fields. The effect of wheat density on wild oat competition with more or less competitive wheat cultivars (Triticum aestivum) was investigated at the Experimental Farm of Plant Pests and Diseases Research Institute, Karaj, Iran. The experiment was established as a factorial combination of wheat varieties Roshan as non-competitive and Niknejad as competitive; three wheat densities (recommended density, recommended+25%, and recommended+50%, corresponding respectively with 300, 375, and 450 plants m-2 for Roshan and 400, 500, and 600 plants m-2 for Niknejad) and four wild oat densities (0, 25, 50, and 75 plants m-2) were selected for this experiment. Hyperbolic equations were used to describe relationship between yield and weed density. Increase in wheat density reduced wild oat biomass. Maximum wild oat biomass was achieved at the highest density of the wild oat together with the lowest wheat density. The results showed that higher densities of wheat are able to suppress wild oat dry matter production. Inter-specific competition in Niknejad was 1.7 times more than that in Roshan. Maximum yields of Niknejad and Roshan in the presence of wild oat were obtained at recommended density+25% and recommended density, respectively. Increase in wheat density leads to a decrease in wheat yield due to an increase in intra-specific competition. Increase in wild oat density results in the reduction of wheat yield through decrease in fertile tiller per plant and spike m-2.
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