The combination of a simulation and an empirical model of crop/weed competition to estimate yield loss from Alopecurus myosuroides in winter wheat

An empirical model of yield loss from assessments of relative weed green area (weed green area/weed + crop green area) was fitted to data from a series of competition trials of winter wheat and black-grass (Alopecurus myosuroides Huds.). Because of the different growth rates of the crop and the weed, relative weed green area increased with time. Consequently, the parameter that describes the competitiveness of the weed (the relative damage coefficient) was highly dependent on the time the weed population was assessed. The ability of two simulation models of crop and weed growth to predict the change in this parameter was investigated. The first was a simple thermal time model, which took no account of competition for resources. This model always underestimated the relative damage coefficient at later assessment dates. Its potential for incorporation into a weed management system was, therefore, limited. The second model included a more comprehensive description of competition for light in the period when shading limits plant growth. This model was more successful at predicting the change in the relative damage coefficient. It could potentially be used in future weed management systems to calibrate a generic value of the relative damage coefficient for a specific weed species to a range of assessment dates. (C) 2003 Elsevier B.V. All rights reserved.