Influence of Herbicide Programs on Glyphosate‐Resistant Giant Ragweed (Ambrosia trifida L.) Density, Soybean Yield, and Net Economic Return in Glyphosate‐ and Glufosinate‐Resistant Soybean

Separate field experiments were conducted in 2011 and 2012 to determine the influence of herbicide programs on the density of glyphosate‐resistant (GR) giant ragweed, yield, and net return in glyphosate‐ and glufosinate‐resistant soybean. In all experiments, preplant (PREPLT) applications were followed by (fb) a post‐emergence (POST) herbicide program. In the GR soybean experiment, the highest density of GR giant ragweed observed 11 weeks after planting (WAP) was 244 plants per plot in response to a PREPLT application of glyphosate fb glyphosate plus fomesafen early post‐emergence (EPOST) fb glyphosate late post‐emergence (LPOST). All other programs resulted in 5 or less plants per plot when PREPLT applications contained a glyphosate tank‐mix combination. Similarly, in the glufosinate‐resistant soybean experiment, all herbicide programs resulted in 6 or less plants per plot 11 WAP. Few differences were observed in soybean yield and net return among herbicide programs that included an effective PREPLT treatment for GR giant ragweed control. Programs that contained an effective PREPLT tank‐mix combination resulted in the highest yield and net return. For example, PREPLT herbicide treatments that contained more than one mode of action (MOA) provided net returns of $122.15 to $201.49/acre higher than glyphosate alone in GR soybean. Results from this research indicate that GR giant ragweed can be managed following an effective PREPLT and POST program in either soybean system.