PREDICTING THE COMPETITIVE EFFECTS OF WEED AND CROP DENSITY ON WEED BIOMASS, WEED SEED PRODUCTION AND CROP YIELD IN WHEAT

Competition between winter-sown wheat and Viola arvensis Murray or Pap aver rhoeas L. was studied in two experiments in two successive years. The effects of varying crop and weed density were modelled in terms o f weed biomass over time, weed seed production and crop yield. Biomass model parameters, representing maximum weed biomass and intra and int erspecific competition, were obtained for different assessment dates, enabling biomass levels to be predicted during the two growing seasons . Weed biomass declined, and its maximum level was reached earlier, wi th increasing crop density. Intraspecific competition was higher in th e absence than in the presence of crop, increasing with time and with weed density. Halving the wheat population increased June biomass of V . arvensis by 74% and of P. rhoeas by 63%. Crop yield losses with incr easing weed density were greater with low than with medium and high cr op populations. P. rhoeas was significantly more competitive than V. a rvensis in both years. Weed biomass in 1989 responded more to reductio ns in crop density following the milder winter of 1988/89 than in the previous year; however crop yields were less affected in 1989 due to s ummer drought, restricting late weed growth and competition. Weed seed production was related to weed biomass; the progressive lowering of c rop density increased seed production, and both species were very prol ific in the absence of crop. By combining models, seed production coul d be derived for a given competitive effect on the crop. Threshold wee d populations, based on low weed levels that are not economic to contr ol, could then be equated with the accompanying weed seed production.