Field studies were conducted at four locations over a 2-year period to evaluate the utility of soil-applied herbicides and glyphosate timing for weed control and soybean yield. Pendimethalin, S-metolachlor plus metribuzin, and flufenacet plus metribuzin were applied pre-emergence (PRE) alone or followed by glyphosate applied early postemergence (EPOST), late postemergence (LPOST), or EPOST plus LPOST. Soil-applied herbicides or glyphosate alone failed to control (<45%) broadleaf signalgrass in 2003 due to late-season rainfall, which accounted for a late flush of growth. In 2004, soil-applied herbicides alone controlled 79–100% broadleaf signalgrass, whereas glyphosate alone or in combination with soil-applied herbicides controlled at least 99%. Barnyardgrass and tall waterhemp were controlled at least 87% with soil-applied herbicides alone and at least 95% when glyphosate was used alone or in combination with a soil-applied herbicide. Soybean yield varied, but at only one location did herbicide treatments produce higher yields than the untreated check. Under low to moderate weed pressure, the use of a soil-applied herbicide followed by glyphosate failed to increase net returns over soil-applied herbicides alone.
Nomenclature: Flufenacet metribuzin; glyphosate; S-metolachlor metribuzin; pendimethalin; broadleaf signalgrass; Brachiaria platyphylla (Griseb.) Nash #3 BRAPP; barnyardgrass; Echinochloa crus-galli (L.) Beauv. # ECHCG; tall waterhemp, Amaranthus tuberculatus (Moq.) J. D. Sauer # AMATU; soybean, Glycine max (L.) Merr. ‘HBK 5123RR, DP 5110RR’.
Additional index words: Application window, sequential.
Abbreviations: EPOST, early postemergence; fb, followed by; LPOST, late postemergence; POST, postemergence; PRE, preemergence.
