Glyphosate with and without residual herbicides in no-till glyphosate-resistant soybean (Glycine max)

Field experiments were conducted in 1996 and 1997 near Arlington, WI, to co mpare the efficacy of glyphosate applied below registered rates in sequenti al and tank-mix combinations with residual herbicides to no-till, narrow-ro w, glyphosate-resistant soybean. In the sequential combinations of preplant (PP) residual herbicides and postemergence (POST) glyphosate study, glypho sate followed one of eight burndown treatments. Clomazone applied PP contro lled 86% of common lambsquarters in 1996 when followed by 420 g ae/ha glyph osate early POST (EPOST). All other herbicide treatments controlled 94% or greater regardless of weed species, PP treatment, glyphosate timing, or gly phosate rate. The greatest soybean yields occurred in EPOST glyphosate appl ications in 1996 and late POST (LPOST) glyphosate applications in 1997. The only time PP residual herbicides were beneficial was prior to the LPOST gl yphosate application in 1996. In the tank-mix POST combinations of glyphosa te and residual herbicides study, glyphosate was applied alone or in combin ation with four residual herbicides. Soybean injury did not exceed 5% excep t in the glyphosate and imazethapyr combination in 1997. Control of common lambsquarters, velvetleaf, and giant foxtail was 90% or greater when averag ed across all residual combinations and glyphosate timings and rates. Imaze thapyr 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 result ed in a lower yield than 420 g/ha glyphosate LPOST Only one residual herbic ide, SAN 582, combined with glyphosate produced yields equivalent to the hi ghest yielding treatments when averaged over both glyphosate rates and timi ngs. Cloransulam added to 420 g/ha glyphosate EPOST and chlorimuron plus th ifensulfuron and imazethapyr added to 420 gl ha glyphosate LPOST resulted i n lower soybean yields compared to the same rate of glyphosate applied alon e at the respective timings. Thus, no herbicide combination preformed bette r than glyphosate applied in a timely manner alone. However, in situations where early-season weed competition is severe and a timely glyphosate appli cation is not possible, a PP residual herbicide may be beneficial.