Modeling competition for light between soybean and broadleaf weeds

Abstract Crop yield reductions due to weed competition can vary with both environmental and cultural conditions. Models of resource use by crops and weeds will likely be necessary to accurately simulate crop losses from weed competition. Many broadleaf weeds cause losses in soybean ( Glycine max (L.) Merr.) through competition for light. A simple model of canopy structure and light interception by soybean and broadleaf weeds within a weed area of influence has been developed and incorporated into SOYWEED, a dynamic soybean-weed competition model. Daily photosynthetic rates per unit ground area are calculated according to interception of direct light by the weed and crop. Interception is approximated from the simple exponential extinction of light by a plant canopy. For competing plants, interception depends on both the amount and arrangement of leaf area. Arrangement of that leaf area is described by extinction coefficients, the plant height, and the vertical distribution of leaf area. Preliminary testing with cocklebur ( Xanthium strumarium L.) competition data indicates an improved simulation of weed and crop growth and crop yield with the incorporation of this model into SOYWEED, but highlights the lack of data on weed and soybean canopy structure. Simulated crop response to changes in weed emergence date and time before weed removal is consistent with general observations reported in the literature. Simulation results indicate that several weed canopy characteristics may contribute to the differential ability of weed species to compete with soybean.