Emergence, Competition, and Management of Glyphosate-Resistant Common Ragweed (\u3ci\u3eAmbrosia artemisiifolia\u3c/i\u3e L.) in Nebraska Soybean

Common ragweed (Ambrosia artemisiifolia L.) is a competitive annual broadleaf weed in soybean (Glycine max) production fields throughout North America. The recent confirmation of glyphosate-resistant common ragweed in Nebraska justified the need to assess the emergence pattern and competitive ability of common ragweed in soybean and to evaluate alternative herbicide programs for effective management. The objectives of this research were to: 1) evaluate the effect of tillage and develop a predictive model for the emergence pattern of common ragweed in Nebraska; 2) model the competitive interaction between soybean and common ragweed as influenced by density and irrigation levels; 3) characterize the growth response of soybean and common ragweed in mixture and monoculture to varying irrigation levels and increasing common ragweed density; and 4) evaluate the efficacy of preplant herbicides followed by glufosinate applied alone or in tank-mixtures for control of glyphosate-resistant common ragweed in glufosinate-resistant soybean. A field study was conducted for three years to evaluate the effect of tillage timing and develop a predictive model for common ragweed emergence in Nebraska. The results of this study conclude that spring tillage does not stimulate additional emergence; therefore, tillage could be used as a component of glyphosate-resistant common ragweed management programs in Nebraska. Additionally, thermal time calculations with a temperature base of 3 C can be used to predict emergence (%). A field study was conducted to model the competitive interaction and assess the growth response of soybean and common ragweed as influenced by density and irrigation level. Soybean yield loss was not altered by irrigation amount and the leaf area ratio model at the soybean R6 growth stage best fit the data. Common ragweed densities of 1, 6, and 12 m─1 row resulted in yield losses of 61, 76, and 95% in 2015 and 25, 39, and 80% in 2016, respectively. Soybean growth was affected by common ragweed density however soybean demonstrated no plasticity. Common ragweed growth was affected by common ragweed density and irrigation. Common ragweed demonstrated plasticity by altering specific leaf area and biomass partitioning when in competition with soybean. A field study was conducted to evaluate the efficacy of glufosinate-based herbicide programs for season-long control of glyphosate-resistant common ragweed in glufosinate-resistant soybean. The results of this study conclude that glufosinate, paraquat, 2,4-D, dimethenamid-P, cloransulam-methyl, or plus chlorimuron ethyl applied preplant (PP) followed by glufosinate applied POST alone or in tank-mixture provided ≥ 84% control of glyphosate-resistant common ragweed, reduced density to ≤ 20 plants m─2, and secured ≥ 1819 kg ha─1 soybean yield. Preplant followed by POST resulted in the highest gross profit margins compared to PP alone or PRE followed by POST treatments