A 3-yr field study was conducted from 2005 to 2007 at Stoneville, MS, to determine efficacy of in-crop and autumn-applied glyphosate on purple nutsedge density and yield of no-till glyphosate-resistant (GR) corn and GR soybean. Separate experiments were conducted in GR corn and GR soybean in areas maintained under a no-tillage system after the autumn of 2004. Each experiment was conducted in a split-plot arrangement of treatments in a randomized complete-block design with and without autumn application of glyphosate at 1.68 kg ae/ha as main plots and in-crop herbicide application (glyphosate- and nonglyphosate-based programs) as subplots with three replications. In GR corn, glyphosate applied in the autumn reduced purple nutsedge density by 40 to 67% compared with no glyphosate during 3 yr. In GR corn, glyphosate applied in-crop reduced purple nutsedge shoot density by 48% in 2005, 92% in 2006, and 100% in 2007 compared with no herbicide. However, GR corn yields were unaffected by either in-crop or autumn-applied glyphosate. In GR soybean, glyphosate applied in the autumn reduced purple nutsedge shoot density by 64 to 83% compared with no glyphosate during 3 yr. Glyphosate applied in-crop in GR soybean reduced purple nutsedge density by 81% in 2005 and by 100% in 2006 and 2007 compared with no herbicide. GR soybean yields were similar in 2005, but yields were 34 and 18% higher in 2006 and 2007, respectively, with autumn-applied glyphosate compared with no glyphosate. GR soybean yields were higher with glyphosate applied in-crop compared with no herbicide in 2 of 3 yr. These results indicate that purple nutsedge density could be reduced with glyphosate applied in-crop in no-till GR corn and GR soybean. In addition, autumn-applied glyphosate was effective in reducing purple nutsedge populations following harvest of crops and could be an effective purple nutsedge management strategy regardless of GR trait.

Nomenclature: Glyphosate; chlorimuron; halosulfuron; S-metolachlor; purple nutsedge, Cyperus rotundus L. CYPRO; corn, Zea mays L.; soybean, Glycine max (L.) Merr.