Weed Control in Roundup Ready® Cotton: Preliminary Experiments

Abstract

The efficacy of Roundupm herbicide for the control of common Arizona weeds in Roundup Ready® cotton was evaluated in field studies conducted at the Maricopa Agricultural Center in 1995. Promising results were obtained with 1 lb a.i./A (1 quart/A) over-the-top band applications of Roundup when cotton had 1 to 2 true leaves followed by a second, "sloppy", post-directed application ofRoundup at 1 lb a.i./A when cotton had 4 to 5 true leaves. Excellent control of small Palmer Amaranth, Wright Groundcherry, and annual morningglory seedlings was obtained with each 1 lb a.i./A Roundup application in the broadleaf weed study. The two sequential 1 lb a.i./A Roundup applications kept plots free of broadleaf weeds until layby. In the nutsedge weed control study, it was found that nutsedge plants treated with a single 1 lb a. i./A application of Roundup were stunted and exhibited leaf chlorosis, but efficacy was marginal with weed control ratings of only 40 to 50 percent. Nutsedge control ratings for an application of norflurazon (i.e., Zorial) alone or combined with Roundup applied at the 1 to 2 true leaf stage of cotton were 63 and 75 percent, respectively. Two sequential applications of Roundup at 1 lb a.i./A, both with or without norflurazon, resulted in about 90 percent nutsedge control. Roundup Ready cotton exhibited good tolerance to two sequential early season 1 lb a.i./A Roundup applications, as made in these studies, with no apparent seed cotton or lint yield reductions.