Field studies were conducted over 2 yr to evaluate weed control, yield, and net returns of glyphosate-resistant soybean using total postemergence (5 wk) (POST) herbicide systems with glyphosate–isopropylamine (Ipa) or glyphosate–trimethylsulfonium (Tms) alone, tank mixed with fomesafen, or in sequential treatments with bentazon, fomesafen, Ipa, or Tms. Soybean early-season injury ranged from 0 to 28% across the test. Although Ipa did not injure soybean, glyphosate–Tms early postemergence (3 wk) (EPOST) injured soybean from 7 to 17% depending on the rate. Glyphosate–Tms mixed with fomesafen EPOST injured soybean from 20 to 28%. Red morningglory control by Ipa and Tms at 0.8 kg ae/ha was no more than 88%. Sequential applications of Tms or Ipa controlled red morningglory 78% or less. Fomesafen improved red morningglory control by Ipa and Tms. Bentazon did not affect the control of red morningglory by these herbicides. Sicklepod, smooth pigweed, and large crabgrass control was 81, 93, and 79%, respectively, or greater for all herbicide treatments. By midseason, narrow-row soybeans had canopied, and competition from weeds was minimal. Overall, the net returns were reflective of soybean yield, and maximum net returns were recorded for treatments with reduced herbicide inputs. Conversely, sequential application of herbicides as EPOST followed by POST treatments resulted in lower net returns because of increased herbicide and application costs.
Nomenclature: Bentazon; fomesafen; glyphosate–isopropylamine; glyphosate–trimethylsulfonium; large crabgrass, Digitaria sanguinalis (L.) Scop. #3 DIGSA; red morningglory, Ipomoea coccinea L. # IPOCC; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby # CASOB; smooth pigweed, Amaranthus hybridus L. # AMACH; soybean, Glycine max (L.) Merr.
Additional index words: AMACH, Amaranthus hybridus, CASOB, Digitaria sanguinalis, DIGSA, Glycine max, herbicide-resistant crops, IPOCC, Ipomoea coccinea, net returns, Roundup-Ready® soybean, Senna obtusifolia, soybean injury, soybean yield, sulfosate.
Abbreviations: EPOST, early postemergence (3 wk); fb, followed by; Ipa, isopropylamine; POST, postemergence (5 wk); Tms, trimethylsulfonium.
