TECHNIQUES TO ESTIMATE RELATIVE LEAF-AREA AND COVER OF WEEDS IN CROPS FOR YIELD LOSS PREDICTION

Destructive measurements to collect input data for models that predict yield loss from relative leaf area of weeds can be laborious. Alternative methods were tested in seven field experiments with sugar beet or spring wheat. Weeds with different morphologies showed the same linear relationships between relative leaf area, measured destructively, and cover, assessed by means of a frame, until 3 or 4 weeks after crop emergence. At later growth stages, differences in weed morphology resulted in different relationships. Visual estimates of weed cover corresponded only roughly with cover assessments with a frame. The possibility of estimating relative leaf area of weeds with a reflectance technique was tested, assuming that for early growth stages the leaf area index of weeds can be considered as additional to that of the crop. In spring wheat. relative leaf areas of Sinapis alba L., sown at different times and densities, correlated well with characteristics based on infra-red reflectance. In sugar beet, these relationships were not as distinct.