Common Ragweed (Ambrosia artemisiifolia L.) Interference with Soybean in Nebraska

Common ragweed is a competitive weed in soybean fields in north central United States and eastern Canada. The effect of available soil water on the competitiveness of common ragweed in soybean hasn't been determined. A field study was conducted in 2015 and 2016 in Nebraska to assess common ragweed interference in soybean as affected by available soil water and common ragweed density. The experiment was arranged in a split-plot design with irrigation level as main plots and common ragweed density as subplots. Periodic crop and weed leaf area index (LAI) and aboveground biomass were sampled and soybean yield was harvested. A model set was constructed using the rectangular hyperbolic and leaf area ratio models and the best model for predicting yield loss among years was identified using the information-theoretic criterion. No effect of irrigation level on soybean yield was detected due to near adequate rainfall during the study. Common ragweed densities of 2, 6, and 12 m(-1) row resulted in soybean yield losses of 76, 91, and 95% in 2015 and 40, 66, and 80% in 2016, respectively. The leaf area ratio model using relative leaf area at the R6 growth stage of soybean best fit the data. The leaf area ratio model includes both soybean and common ragweed leaf area and, therefore, is putatively more effective at accounting for variation in competition for light than density among years. Results of this study suggest that soybean-common ragweed interference resulted in substantial soybean yield loss when competing for light.