Preemergence weed control in soybean with cloransulam

Field experiments were conducted from 1999 to 2001 to evaluate preemergence (PRE) activity of cloransulam on broadleaf weed species and to determine the effectiveness of cloransulam as a PRE herbicide in glyphosate-resistant soybean weed management systems. Cloransulam PRE controlled prickly sida, velvetleaf, and morningglory species even at reduced rates (recommended rate 36 g ai/ha) but only suppressed growth of Palmer amaranth, hemp sesbania, and sicklepod. Cloransulam applied PRE provided initial control or suppression of most weeds, but late-season control declined appreciably. Adding metribuzin to cloransulam PRE generally improved control of hemp sesbania, Palmer amaranth, annual grasses, and morningglory species, leading to soybean yield increases. Control of weeds was greater on a silt loam soil compared with a silty clay soil. Delayed herbicide activation by rainfall or irrigation reduced control of hemp sesbania and prickly sida and affected efficacy more than soil texture. Single postemergence (POST) applications of glyphosate or fomesafen plus fluazifop-P provided 90% or less control of most weed species. When glyphosate POST or fomesafen plus fluazifop-P POST followed PRE applications of cloransulam or cloransulam plus metribuzin PRE, control of all weeds was generally greater than 85%. The highest soybean yields were recorded from treatments that contained sequential PRE followed by (fb) POST herbicide applications. Composition of weed flora determined the effect of herbicide program on soybean seed yield. No yield benefit was gained from the sequential program when the dominant species was Palmer amaranth, which was controlled by glyphosate. When hemp sesbania was the dominant species, PRE herbicides fb glyphosate POST increased yield compared with total POST glyphosate.