Weed Control for Doublecrop Soybeans Planted with the No‐Tillage Method Following Small Grain Harvest1

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Abstract
The no‐tillage method is used extensively to produce doublecrop soybeans [Glycine max (L.) Merr.] planted after winter grain harvest. Success of this method depends on proper selection and effective application of herbicides for weed control in the second crop. Herbicides were evaluated in the field during three seasons for doublecrop soybean production following winter wheat (Triticum aestivum L.) harvest. Both residual herbicides and herbicides having activity only when applied to the foliage of green plants were evaluated alone and in various combinations in this study. The design of the experiment allowed a partitioning of the effects of herbicides having activity only when applied to foliage and utilized to control weeds present in the wheat stubble from the effects of residual herbicides employed to control growth of weeds emerging after soybean planting and herbicide application. During two seasons, herbicides having activity only when applied to foliage accounted for one‐half and two‐thirds of the yield response to weed control. Residual herbicides accounted for the other part of the yield response. In the third season, very few weeds were present in the wheat stubble at harvest and herbicides active only when applied to foliage had no effect on yield. During this season, effective residual herbicides increased yields significantly. Residual herbicide combinations that included linuton [3‐(3,4‐dichlorophenyl)‐l‐methoxy‐l‐methylurea] and metribuzin [4‐amino‐6‐tert‐butyl‐3‐(methylthio)‐as‐triazin‐ 5(4H)‐one] were most consistent for weed control and crop yield. Both glyphosate [N‐(phosphonomethyl)glycine] and paraquat (1,1′‐dimethyl‐4,4′‐bipyridinium ion) generally were satisfactory as herbicides used to control existing vegetation. Some of the residual herbicides used in this study also exhibit considerable activity on foliage of plants present but at rates used this was inadequate for maximum crop yields.
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