Diuron [3‐(3,4‐dichlorohenyl‐l,l‐dimethylurea] has been used for many years to control Italian ryegrass (Lolium multiflorum Lam.) in winter wheat (Triticum aestivum L.) in western Oregon. Variable results cannot be attributed solely to soil and climatic factors. Studies were conducted to determine the effect of ryegrass density on diuron efficacy. Under both greenhouse and field conditions, control of ryegrass decreased as plant density was increased. One possible explanation for lower efficacy in dense stands is the reduced amount of herbicide in the soil solution available for each plant. The influence of density on herbicide performance may play an important role in commercial field practice as well as in the interpretation of bioassay results.
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Burrill et al. (1978) studied this question.