Field studies were conducted in 1999 and 2000 at Belleville, IL, to evaluate herbicide efficacy, grain yield, and economic return on investment (EROI) from various herbicide systems in conventional, sulfonylurea-tolerant, glufosinate-resistant, and glyphosate-resistant soybean varieties. Several grower-oriented herbicide systems were developed within each soybean variety using current weed control options. These herbicide systems included herbicides with soil-residual, herbicides with different modes of action, and combinations of preemergence (PRE) followed by (fb) postemergence (POST) herbicides. Soybean injury ranged from 1 to 16% 28 d after (DA) PRE applications of sulfentrazone plus chlorimuron plus pendimethalin. Injury 7 DA POST treatments ranged from 2 to 39%, with the greatest injury occurring from the conventional herbicide combination of fomesafen plus fenoxaprop plus fluazifop-P. No injury symptoms persisted through soybean harvest. Control of giant foxtail was the most consistent (97% or greater) in the PRE broadleaf fb POST herbicide system 56 DA planting. Common waterhemp control was at least 85% for all treatments, with the exception of the single POST herbicide systems of chlorimuron plus thifensulfuron plus quizalofop-P, and glufosinate and the standard sequential herbicide system of pendimethalin fb chlorimuron plus thifensulfuron. All treatments provided at least 87% control of common ragweed and ivyleaf morningglory. Soybean grain yield was similar between varieties in the weed-free control plots in 1999. In 2000, grain yield of the glyphosate- and glufosinate-resistant soybean varieties was greater than for the conventional and sulfonylurea-tolerant varieties. Overall, soybean grain yield and EROI were less for some treatments because of poor control of common waterhemp. Weed control had a greater influence than herbicide-related costs on EROI.
Nomenclature: Chlorimuron; fenoxaprop; fluazifop-P; fomesafen; glufosinate; glyphosate; pendimethalin; quizalofop-P; sulfentrazone; sulfonylurea; thifensulfuron; common ragweed, Ambrosia artemisiifolia L. # AMBEL; common waterhemp, Amaranthus rudis Sauer. #3 AMATA; giant foxtail, Setaria faberi Herrm. # SETFA; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; soybean, Glycine max (L.) Merr.
Additional index words: Glufosinate-resistant, glyphosate-resistant, sulfonylurea-tolerant.
Abbreviations: ALS, acetolactate synthase; DA, days after; DAT, days after treatment; EROI, economic return on investment; fb, followed by; POST, postemergence; PRE, preemergence.