Recommended field application rates of herbicides have to give effective weed control in every situation and are, thus, often higher than that required for specific fields. An understanding of the interaction between crop:weed competition and herbicide dose may, in many cases, allow herbicide application 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 crop yield and weed biomass (related to weed density), and an empirical model of the relationship between weed biomass and herbicide dose. The combined model predicts crop yield, given herbicide dose and weed biomass at an interim assessment date. These crop yield loss predictions may be used to quantify the herbicide dose required to restrict yield loss to a given percentage. Parameters of the model were estimated and the model tested, using results from experiments, which used cultivated oats (Avena sativa) or oilseed rape (Brassica napus) as model weeds in a crop of winter wheat (Triticum aestivum).For the crop:weed:herbicide combinations investigated there was little increase in crop yield for herbicide dose rates above 20% of recommended field rates, in broad agreement with the model predictions. There may still be potential for further reduction below this level on economic grounds; the model could be used to estimate the `break‐even' herbicide dose.

Seeds of 40 populations of Bromus sterilis L. were collected in the southern and midland counties of England over a 2-year period and grown outdoors in pots in two subsequent years. In the ®rst year, seeds were tested in a 12-h dark/12-h light regime at 15°C and in the second year the seeds were tested both in the dark/light regime and in the dark at 15°C. There was a wide range in the degree of enforced dormancy given by the dark/light regime. Germination of freshly collected seeds in the dark/light regime after 21 days ranged from 44% to 97% in the populations tested in the ®rst year and from 19% to 97% in populations tested in the second year. Induced dormancy was caused by light in two populations. Seeds had little innate dormancy apart from in two populations which gave 64% and 68% germination, respectively, in the dark after 21 days. A ®eld trial in which seeds of a selected range of six populations were sown on the soil surface after harvest (August 8), showed that populations predicted to be inhibited by light in laboratory tests were also inhibited by light in the ®eld and, depending upon the population, there was between 4% and 54% of the seeds remaining ungerminated by October 23 in the year of planting. By June of the following year, 36% of the seeds sown on the soil surface in one population still remained viable and ungerminated. The agricultural signi®cance of the results is discussed.