Abstract: Field experiments were conducted in 1997 and 1998 near Columbia and Novelty, MO, and Urbana, IL, to evaluate crop injury, weed control, corn yield, and net economic returns provided by weed control programs in glyphosate-resistant corn. The herbicide programs evaluated included acetochlor preemergence (PRE) followed by (fb) glyphosate with or without atrazine postemergence (POST) and total POST programs consisting of single and sequential applications of glyphosate alone and tank-mixed with actochlor, atrazine, or both. Metolachlor PRE fb dicamba plus atrazine POST and metolachlor plus atrazine PRE were included for comparison. In the total POST treatments, mid-post (MPOST) applications provided better control than early-post (EPOST) applications on weeds that germinated throughout the growing season such as shattercane and common cocklebur, but also resulted in yield reductions of up to 23% caused by early-season weed competition. The addition of atrazine to glyphosate POST generally increased control of common cocklebur, morningglory species, and common waterhemp. EPOST or PRE fb EPOST applications generally provided higher yields than MPOST treatments, although MPOST treatments often provided equal or greater weed control at midseason. Treatments including two herbicide applications tended to provide greater weed control, yield, and profit than those with a single application. Input costs for glyphosate-resistant corn are slightly higher than nontransgenic hybrids. However, net economic returns are similar and the use of glyphosate POST allows greater flexibility in POST weed management decisions.
Nomenclature: Acetochlor, 2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide; atrazine, 6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine; dicamba, 3,6-dichloro-2-methoxybenzoic acid; glyphosate, N-(phosphonomethyl)glycine; metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide; common cocklebur, Xanthium strumarium L.#3 XANST; common waterhemp, Amaranthus rudis Sauer # AMATA; ivyleaf morningglory, Ipomoea hederaceae (L.) Jacq. # IPOHE; pitted morningglory, Ipomoea lacunosa L. # IPOLA; shattercane, Sorghum bicolor (L.) Moench. # SORVU; corn Zea mays L. # ZEAMX ‘MON802’.
Additional index words: Corn yield, acetochlor, atrazine, dicamba, economic returns, metolachlor, AMATA, ABUTH, CHEAL, IPOHE, IPOLA, POLPY, SETFA, SORVU, XANST, ZEAMX.
Abbreviations: ALS, autolactate synthase; EPOST, early postemergence; fb, followed by; MPOST, mid-postemergence; OM, organic matter; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.