Horseweed (Conyza canadensis) Control in No-Tillage Soybeans (Glycine max) with Preplant and Preemergence Herbicides

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Abstract
Research was conducted in 1987 and 1988 to identify effective horseweed control strategies in no-tillage soybean production. Paraquat at 560 g ai ha-1 plus metolachlor at 2200 g ai ha-1, linuron at 840 g ai ha-1, and nonionic surfactant at 0.25% (v/v) applied preemergence provided less than 61% horseweed control. Glyphosate at 840 g ai ha-1, 2,4-D ester at 560 g ai ha-1, HOE-39866 at 840 g ai ha-1, or BAS-514 at 70 g ai ha-1 applied early preplant controlled more than 96% when followed by the above preemergence herbicide combination. Substituting either glyphosate at 840 g ha-1 or HOE-39866 at 840 g ha-1 for paraquat in the preemergence application program significantly increased control. Adding BAS-514 at 70 g ha-1 to preemergence treatments containing paraquat significantly improved control. The substitution of metribuzin, metribuzin plus chlorimuron (10:1 ratio), or linuron plus chlorimuron (16:1 ratio) for linuron resulted in greater control and soybean yield. Sequential applications controlled horseweed equal to or better than a single preemergence application at the same total rate.