Predicting the growth and competitive effects of annual weeds in wheat

Summary: The growth and competitiveness of 12 annual weed species were studied in crops of winter wheat, in which weeds were sown to give a wide range of plant densities. Weed growth patterns were identified; early species which senesced in mid‐summer were less competitive than those with a growth pattern similar to that of the crop. Most species had little effect on crop yield in 1987, and this was attributed to a high crop den sity. Crop yield‐weed density relationships for all species in 1988 and for Galium aparine in 1987 were well described by a rectangular hyperbola. Species were listed in the following competitive order based on the percentage yield loss per weed m −2 : Avena fatua > Matricaria perforata > Galium aparine > Myosotis arvenis > Poa trivialis > Alopecurus myosuroides > Stellaria media > Papaver rhoeas > Lamiumpur‐pureum > Veronica persica > Veronica hederi‐folia > Viola arvensis . Prediction of yield loss is discussed. The assumptions inherent in using Crop Equivalents (based on relative weights of weed and crop plants), are challenged; with intense competition, weed biomass at harvest failed to replace lost crop biomass, and harvest index was reduced. It is concluded that a competi tive index, derived from yield density relation ships, and expressed as the percentage yield loss per weed m −2 , is more likely to reflect the com petitive ability of a species than an index obtained from plant weights in the growing crop.