Interaction of glyphosate with postemergence soybean (<i>Glycine max</i>) herbicides

Abstract

Greenhouse and field experiments were conducted to evaluate the potential for antagonistic or synergistic interactions from tank mixtures of glyphosate plus a selective herbicide applied postemergence. In the greenhouse, glyphosate at 420 g ae ha −1 plus 28% liquid urea-ammonium nitrate (28% UAN) provided at least 89% control of common lambsquarters and common ragweed. Glyphosate at 1,680 g ha −1 plus 28% UAN provided less than 81% control of velvetleaf and less than 75% control of ivyleaf morningglory. Tank mixing bentazon at 1,120 g ai ha −1 with glyphosate at 420 g ha −1 synergistically increased control of velvetleaf. Tank mixtures of glyphosate plus a selective herbicide were predominately additive in control of common lambsquarters, common ragweed, and velvetleaf. Several tank mix combinations of chlorimuron or imazethapyr plus glyphosate plus 28% UAN were antagonistic in control of ivyleaf morningglory. In the field, glyphosate at 840 g ha −1 plus 28% UAN provided at least 88% control of common lambsquarters and velvetleaf in 1994. However, glyphosate at 840 g ha −1 plus 28% UAN provided only 60% control of velvetleaf in 1995. Tank-mixing bentazon or CGA-248757 with glyphosate at 420 g ha −1 increased velvetleaf and common lambsquarters control in 1995. In general, adding chlorimuron, imazethapyr, or thifensulfuron to glyphosate plus 28% UAN did not increase control of common lambsquarters or velvetleaf. Tank mixing imazethapyr with glyphosate plus 28% UAN antagonized velvetleaf control in 1994 and in 1995. The tank mixture of thifensulfuron at 2 g ha −1 plus glyphosate at 420 g ha −1 plus 28% UAN increased soybean injury in the field in 1994. However, tank mixing chlorimuron, imazethapyr, or thifensulfuron with glyphosate plus 28% UAN did not increase soybean injury in the greenhouse or in the field in 1995.