The effects of a range of herbicide doses on crop:weed competition were investigated by measuring crop yield and weed seed production. Weed competitivity of wheat was greater in cv. Spark than in cv. Avalon, and decreased with increasing herbicide dose, being well described by the standard dose–response curve. A combined model was then developed by incorporating the standard dose–response curve into the rectangular hyperbola competition model to describe the effects of plant density of a model weed, Brassica napus L., and a herbicide, metsulfuron‐methyl, on crop yield and weed seed production. The model developed in this study was used to describe crop yield and weed seed production, and to estimate the herbicide dose required to restrict crop yield loss caused by weeds and weed seed production to an acceptable level. At the acceptable yield loss of 5% and the weed density of 200 B. napus plants m–2, the model recommends 0.9 g a.i. metsulfuron‐methyl ha–1 in Avalon and 2.0 g a.i. in Spark.

The effects of nitrogen fertiliser on herbicide doseresponse of weeds were investigated by measuring weed biomass after growth at a range of nitrogen levels and treatment with a range of herbicide doses. Increasing weed biomass at no-herbicide treatment (W 0 ) and the response rate of the dose-response curve (B), with increasing nitrogen were successfully described by the linear model and the exponential model respectively. Conversely, decreasing ED 50 value with increasing nitrogen was well described by the logistic model. A combined model was then developed by incorporating these models into the standard dose-response model to describe the interactive effects of herbicide dose and nitrogen levels on biomass of Brassica napus, Matricaria perforata, Papaver rhoeas and Galium aparine. The model developed allowed the systematic description of increased herbicide performance with increasing nitrogen. The model was also used to predict weed biomass as affected by both herbicide doses and nitrogen levels. The mathematical relationships between herbicide dose-response and nitrogen levels may also be applied to the crop-weed competition model and then to decision making for optimum uses of nitrogen fertiliser and herbicide.