SIMULATION OF COMPETITION FOR PHOTOSYNTHETICALLY ACTIVE RADIATION BETWEEN COMMON RAGWEED (AMBROSIA-ARTEMISIIFOLIA) AND DRY BEAN (PHASEOLUS-VULGARIS)

The influence of weeds on crop yield is not only dependent on weed-rel ated factors such as density and time of emergence, but also on enviro nmental and management factors that affect both the weed and crop thro ugh time. This study was undertaken to develop the first physiological ly based dry bean model that would account for the influence of weed c ompetition. The specific objective was to develop a model that would a ccount for the influence of weed competition on crop yield, and to use this model to test the hypothesis that crop yield losses resulted fro m competition for photosynthetically active radiation (PAR). To this e nd, a model that simulated the growth and development of dry bean was developed. The model performed daily calculations and simulated the ph enology, leaf area expansion, dry matter production and distribution, and grain yield of dry bean based on weather and management informatio n, but assumed adequate water and nutrients. The model was calibrated without weed competition at two locations and yr, and for these situat ions, adequately described the growth and development of the crop. Sim ulations were then run for five common ragweed densities and two times of emergence, Common ragweed leaf area was read into the model from i nput files and used to simulate weed shading. Shading of the dry bean canopy by common ragweed accounted for about 50 to 70% of the yield lo sses observed in field studies when weeds emerged with the crop. Weed shading did not account for the yield reduction measured from weeds th at emerged at the second trifoliate stage of crop growth. The agreemen t between model predictions and field studies was consistent with the hypothesis that competition for PAR was a principal factor in weed-cro p interaction. The ability to account for differences in weed densitie s, management, and environmental conditions suggested that modeling wa s a useful tool for evaluating the interaction among weeds and crops.