A model for light competition between vegetable crops and weeds

A precise prediction of the yield losses inflicted by weeds is the basis of decisions in weed management. Hitherto, only rough estimates, which neglect the specific production situation, have been available for vegetable crops. In this study a simple simulation model was developed to estimate yield loss by radiation competition as a function of environmental variables. In the model, the distribution of incoming photosynthetically active radiation (PAR) in the canopy is calculated using a spatially highly resolved approach. Growth is calculated as a function of absorbed radiation and its utilisation. Newly produced dry matter is allocated to roots and shoots, the latter comprising vegetative and reproductive organs according to the developmental stage. Vegetative shoot dry matter is partitioned according to the main functions of radiation interception (leaves) and structural stability (stems and petioles). The resulting leaf area is distributed in the canopy according to the spatial expansion of individual plants. Calibration runs revealed uncertainties predicting the growth of Chenopodium album and a high sensitivity of crop yield to leaf area development of the weed. Using the area of green leaves (LAI) of C. album as input gave a close correspondence between simulated and observed crop yield loss. Since plant height of C. album is calculated as a function of leaf area, this variable has a multiple effect on radiation absorption. A first evaluation with an independent data set likewise gave an acceptable prediction. To reduce model complexity, a simplified version is proposed.