Modelling the effect of crop and weed on herbicide efficacy in wheat

Recommended field application rates of herbicides have to give effective we ed control in every situation and are, thus, often higher than that require d for specific fields. An understanding of the interaction between crop:wee d competition and herbicide dose may, in many cases, allow herbicide applic ation rates to be reduced, important both environmentally and economically. We have developed a model of the interaction between crop:weed competition and herbicide dose, using an empirical model of the relationship between c rop yield and weed biomass (related to weed density), and an empirical mode l of the relationship between weed biomass and herbicide dose. The combined model predicts crop yield, given herbicide dose and weed biomass at an int erim assessment date. These crop yield loss predictions may be used to quan tify the herbicide dose required to restrict yield loss to a given percenta ge. Parameters of the model were estimated and the model tested, using resu lts from experiments, which used cultivated oats (Avena sativa) or oilseed rape (Brassica napus) as model weeds in a crop of winter wheat (Triticum ae stivum). For the crop:weed:herbicide combinations investigated there was li ttle increase in crop yield for herbicide dose rates above 20% of recommend ed field rates, in broad agreement with the model predictions. There may st ill be potential for further reduction below this level on economic grounds ; the model could be used to estimate the 'break-even' herbicide dose.