Abstract: Field studies were conducted in 19-cm-row soybean during 1997 and 1998 to evaluate the rate and timing of glyphosate applications applied alone at 0.42 and 0.63 kg ae/ha and with other herbicides to glyphosate-resistant soybean in terms of weed control, soybean yield, and net return. Weed species included barnyardgrass, hemp sesbania, pitted morningglory, and prickly sida. Among glyphosate-alone weed control programs, sequential applications generally provided the highest and most consistent control of all species evaluated (above 90% for broadleaf species). Glyphosate applied in combination with selective postemergence (POST) herbicides or following soil-applied herbicides controlled most species comparable to sequential glyphosate applications. Most herbicide programs, except for single glyphosate applications, were comparable to sequential glyphosate applications in terms of soybean yield and net return and ranged from 2,221 to 2,827 kg/ha and $410.28 to $549.60/ha. Depending on the year, either one or two glyphosate applications were needed for adequate weed control in programs with a soil-applied herbicide. Glyphosate rate affected neither weed control, yield, nor net return. Depending on a producer's management style and weed spectrum, weed control programs with glyphosate alone, in combination with another POST herbicide, or following soil-applied herbicides are all viable options in drilled glyphosate-resistant soybean.
Nomenclature: Glyphosate; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; hemp sesbania, Sesbania exaltata (Raf.) Cory. # SEBEX; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP; soybean, Glycine max (L.) Merr. ‘Delta King 5961 RR’.
Additional index words: Glyphosate economics, Echinochloa crus-galli, Ipomoea lacunosa, Sesbania exaltata, Sida spinosa, Glycine max, ECHCG, IPOLA, SEBEX, SIDSP.
Abbreviations: NIS, nonionic surfactant; POST, postemergence.