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