Benefits of Soil-Applied Herbicides in Glyphosate-Resistant Soybean (Glycine max)1

Field research conducted over 3 yr evaluated the utility of preemergence (PRE), soil-applied herbicides at half- and full-label rates in glyphosate-resistant soybean. Soil-applied herbicide treatments at full-label rates included pendimethalin plus imazaquin (0.84 + 0.14 kg ai/ha), pendimethalin (1.12 kg/ha), metolachlor (1.68 kg ai/ha), dimethenamid plus imazaquin (1.0 + 0.14 kg ai/ha), sulfentrazone plus chlorimuron (0.22 + 0.04 kg ai/ha), and metribuzin plus chlorimuron (0.36 + 0.06 kg ai/ha). Weed density and growth were reduced when PRE herbicides were used, and in many cases for broadleaf weeds, half-label rates were as effective as full rates. None of the herbicides provided complete control of all weeds. Sulfentrazone plus chlorimuron reduced ivyleaf morningglory density an average of 90%. For hemp sesbania, metribuzin plus chlorimuron reduced weed emergence over 3 yr at least 95%. The initial glyphosate application was made when the largest weeds, barnyardgrass or hemp sesbania, reached 10 cm. In 1998 all soil-applied herbicide treatments extended the time period of glyphosate application by 3 to 5 d when compared with the nontreated control. In 1999 the full rate of metribuzin plus chlorimuron delayed the application of glyphosate by 6 d, and an extension of 7 d was noted for the full rates of sulfentrazone or metribuzin plus chlorimuron in 2000. When soil-applied herbicides were used each year, only a single application of glyphosate was needed. A second glyphosate application was needed in only 1 yr when soil-applied herbicides were not used. Even though differences in weed control were observed among the herbicide treatments, soybean yield was the same. Nomenclature: Chlorimuron; dimethenamid; glyphosate; imazaquin; metolachlor; metribuzin; pendimethalin; sulfentrazone; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex A. W. Hill # SEBEX; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; prickly sida, Sida spinosa L. # SIDSP; redweed, Melochia corchorifolia L. # MEOCO; soybean, Glycine max (L.) Merr. ‘Asgrow 5901 RR’. Additional index words: Application window, reduced rates, weed emergence, weed growth. Abbreviations: POST, postemergence; PRE, preemergence.

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