PREDICTION OF THE COMPETITIVE EFFECTS OF WEEDS ON CROP YIELDS BASED ON THE RELATIVE LEAF-AREA OF WEEDS

For implementation of simple yield loss models into threshold-based we ed management systems, a thorough validation is needed over a great di versity of sites. Yield losses by competition with Sinapis alba L. (wh ite mustard) as a model weed, were studied in 12 experiments in sugar beet (Beta vulgaris L.) and in 11 experiments in spring wheat (Triticu m aestivum L.). Most data sets were better described by a model based on the relative leaf area of the weed than by a hyperbolic model based on weed density. This leaf area model accounted for (part of) the eff ect of different emerging times of the S. alba, whereas the density mo del did not. A parameter that allows the maximum yield loss to be smal ler than 100% was mostly not needed to describe the effects of weed co mpetition. The parameter that denotes the competitiveness of the weed species with respect to the crop decreased the later the relative leaf area of the mustard was determined. This decrease could be estimated from the differences in relative growth rate of the leaf area of crop and S. alba. However, the accuracy of this estimation was poor. The pa rameter value of the leaf area model varied considerably between sites and years. The results strongly suggest that the predictive ability o f the leaf area model needs to be improved before it can be applied in weed management systems. Such improvement would require additional in formation about effects of abiotic factors on plant development and mo rphology and the definition of a time window for predictions with an a cceptable level of error.