Abstract: Field studies were conducted in 1992 and 1993 to evaluate weed control by 15 herbicide treatments in wheat stubble and in the succeeding corn crop. Atrazine at 2.24 kg ai/ha plus several herbicide treatments were applied about 13, 21, and 33 d following winter wheat harvest on separate plots in 1992 and 1993 in a soybean–winter wheat–corn rotation. Atrazine with and without 2,4-D isooctyl ester at 1.46 kg ae/ha or dicamba at 0.36 kg ae/ha did not control barnyardgrass, green foxtail, horseweed, kochia, stinkgrass, tumble thistle, or witchgrass in the wheat stubble 30 d after treatment. Atrazine mixtures containing glyphosate or paraquat with or without 2,4-D or dicamba controlled most summer annual weed species. Atrazine plus paraquat at 0.43 kg ai/ha was more effective on redroot pigweed and tumble thistle than atrazine plus glyphosate at 0.43 kg ae/ha plus 2,4-D at 0.95 kg ae/ha. Atrazine plus glyphosate mixtures were more effective on barnyardgrass for the first and second application date than atrazine plus paraquat. Increasing the glyphosate rate from 0.43 to 0.67 kg/ha was necessary to control barnyardgrass 95% at the first date. With the first date of application, kochia control was greater when 2,4-D or dicamba was added to the atrazine plus paraquat (0.43 kg/ha) mixture. Although annual grass control was generally greater when weeds approached maturity, early applications are a more sound weed control strategy because of soil water conservation and prevention of weed seed production. However, corn yields in 1993 were greater on plots treated at the third application in 1992 because weed biomass in corn was less. In 1994, corn yields were highest for the first application in 1993, probably because of better weed control in the corn. Above average rainfall in 1993 and 1994 aided corn yields.
Nomenclature: 2,4-D; atrazine; dicamba; glyphosate; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; green foxtail, Setaria viridis (L.) Beauv. # SETVI; horseweed, Conyza canadensis (L.) Cronq. # ERICA; kochia, Kochia scoparia (L.) Schrad. # KCHSC; redroot pigweed, Amaranthus retroflexus L. # AMARE; stinkgrass, Eragrostis cilianensis (All.) E. Mosher # ERACN; tumble thistle, Salsola collina Pall.; witchgrass, Panicum capillare L. # PANCA; corn, Zea mays L. # ZEAMX ‘Wilson 1640’; soybean, Glycine max L. Merr. GLYMA; winter wheat, Triticum aestivum L. # TRAES ‘Arapahoe’.
Additional index words: Conservation tillage, ecofallow, no-tillage, triazine-resistant kochia.
Abbreviations: DAT, days after treatment; NIS, nonionic surfactant; TR, triazine-resistant.
