Field experiments were conducted in 1996 and 1997 near Arlington, WI, to compare the efficacy of glyphosate applied below registered rates in sequential and tank-mix combinations with residual herbicides to no-till, narrow-row, glyphosate-resistant soybean. In the sequential combinations of preplant (PP) residual herbicides and postemergence (POST) glyphosate study, glyphosate followed one of eight burndown treatments. Clomazone applied PP controlled 86% of common lambsquarters in 1996 when followed by 420 g ae/ha glyphosate early POST (EPOST). All other herbicide treatments controlled 94% or greater regardless of weed species, PP treatment, glyphosate timing, or glyphosate rate. The greatest soybean yields occurred in EPOST glyphosate applications in 1996 and late POST (LPOST) glyphosate applications in 1997. The only time PP residual herbicides were beneficial was prior to the LPOST glyphosate application in 1996. In the tank-mix POST combinations of glyphosate and residual herbicides study, glyphosate was applied alone or in combination with four residual herbicides. Soybean injury did not exceed 5% except in the glyphosate and imazethapyr combination in 1997. Control of common lambsquarters, velvetleaf, and giant foxtail was 90% or greater when averaged across all residual combinations and glyphosate timings and rates. Imazethapyr alone controlled velvetleaf 99% and giant foxtail 92% in 1997. When glyphosate was applied alone, soybean yields were similar at all glyphosate rates and application timings, except the 630 g/ha glyphosate LPOST resulted in a lower yield than 420 g/ha glyphosate LPOST. Only one residual herbicide, SAN 582, combined with glyphosate produced yields equivalent to the highest yielding treatments when averaged over both glyphosate rates and timings. Cloransulam added to 420 g/ha glyphosate EPOST and chlorimuron plus thifensulfuron and imazethapyr added to 420 g/ha glyphosate LPOST resulted in lower soybean yields compared to the same rate of glyphosate applied alone at the respective timings. Thus, no herbicide combination preformed better than glyphosate applied in a timely manner alone. However, in situations where early-season weed competition is severe and a timely glyphosate application is not possible, a PP residual herbicide may be beneficial.
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