Variability in the Relationship Between Crop Yield and Weed Density for Winter Wheat and Bromus sterilis

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Abstract
(1) Winter wheat and Bromus sterilis were grown in additive mixtures in two fields at each of two locations in the U.K. Yields and components of yield were measured at harvest for both species. (2) Three non-linear regression models were fitted to the resulting data. The more complex models explained more of the variation in crop yield within fields than did the simpler model. (3) In general, considerable variation in the effect of weed density on crop performance was observed between locations, but not between fields within locations. (4) At low weed densities, grain number per ear of wheat was more affected by B. sterilis than was the number of ears; at high weed densities the opposite was found. Weight of individual grains was increased by the presence of B. sterilis.