Mesotrione Alone and in Mixtures with Glyphosate in Glyphosate-Resistant Corn (Zea mays)1

Field studies were conducted in 1999, 2000, and 2001 to investigate weed control and glyphosate-resistant corn tolerance to postemergence applications of mesotrione at 70, 105, and 140 g ai/ha applied with and without glyphosate at 560 g ai/ha. Mesotrione alone and mixed with glyphosate controlled smooth pigweed greater than 97%% and common lambsquarters 93 to 99%%. Control of common ragweed and morningglory species was variable. Common ragweed control was generally best when mesotrione was applied at 105 or 140 g/ha, and control increased only in 2000 with the addition of glyphosate. Giant foxtail control was below 25%% with all rates of mesotrione, but mixtures of mesotrione plus glyphosate controlled giant foxtail 65 to 75%%. Mesotrione injured glyphosate-resistant corn 4 to 24%% when averaged over glyphosate rates, and injury was usually increased by higher mesotrione rates, with rainfall after herbicide applications, and in mixtures with glyphosate. Injury was transient and did not reduce corn yields. Mesotrione injury on glyphosate-resistant corn was confirmed in the greenhouse, where all mesotrione treatments reduced glyphosate-resistant corn biomass 9 to 23%% compared with the nontreated check.Nomenclature: Glyphosate; mesotrione; common lambsquarters, Chenopodium album L. ##³ CHEAL; common ragweed, Ambrosia artemisiifolia L. ## AMBEL; giant foxtail, Setaria faberi Herrm. ## SETFA; morningglory species, Ipomoea spp. ## IPOSS; smooth pigweed, Amaranthus hybridus L. ## AMACH; corn, Zea mays L. ‘‘Dekalb 626RR’’, ‘‘Dekalb 5863RR’’.Additional index words: IPOHE, IPOLA, Ipomoea hederacea (L.) Jacq., Ipomoea lacunosa L., Ipomoea purpurea L. Roth, IPOPU, total postemergence, transgenic crops, triketone herbicide.Abbreviations: DAT, days after treatment; POST, postemergence; WAT, weeks after treatment.