Growth Stage Affects Dose Response of Selected Glyphosate-Resistant Weeds to Premix of 2,4-D Choline and Glyphosate (Enlist DuoTM Herbicide*)

With the intent to control glyphosate–resistant and hard to control weeds, a formulation of 2,4-D choline (24.4%) and glyphosate (22.1%) (Enlist DuoTM herbicide) (Note 1) has been developed recently to be used post-emergence in corn and soybean tolerant to Enlist DuoTM in the United States. Dose response studies were conducted under greenhouse conditions for the evaluation of effective rates of Enlist DuoTM to control glyphosate-resistant common waterhemp (Amaranths rudis Sauer), giant ragweed (Ambrosia trifida L.), and kochia [Kochia scoparia (L.) Schrad] and to determine the effect of growth stage of these weeds on the efficacy of Enlist DuoTM. Three parameter log-logistic models were used to develop dose response curves. Glyphosate-resistant giant ragweed was the most sensitive of the three weed species, followed by common waterhemp, and kochia. Based on the visual control or injury estimates, the Enlist DuoTM rates required for 90% control (ED90) of common waterhemp, giant ragweed, and kochia were 1179, 825, and 4,382 g ae ha, respectively, for 10-cm tall plants compared to 2,480, 1,101, and 5,305 g ae ha, respectively, for 20-cm tall plants at 21 days after treatment (DAT). The ED90 values calculated on the basis of percent shoot biomass reduction and visual control or injury estimates were usually similar at 21 DAT. The greenhouse studies indicate that Enlist DuoTM can effectively control less than or equal to 20-cm tall glyphosate-resistant giant ragweed and less than or equal to 10-cm tall glyphosate-resistant common waterhemp at the recommended rate (1,640 g ae ha).