Competitive interactions of wheat and wild oats in relation to variations in plant density of both species were investigated in field studies under irrigation and added fertilizer. Competition studies included comparisons by both additive and replacement series. The additive series was used to study plant responses to competition under fixed density of wheat and increasing density of wild oats. The replacement series was used to study plant responses to competition under constant total plant density with differing proportions of wheat and wild oats. On a per plant basis, shoot dry weight and leaf area index of wild oats were less than those of wheat at anthesis. However, the replacement series experiment indicated that wheat and wild oats were equivalent in competitiveness. Relative density of wild oats gave a better regression fit for wheat yield than did absolute wild oats density (r2 values of 0.83 and 0.61, respectively). Yield of wheat grain was linearly proportional to relative density of wild oats.
