Zinc Antagonizes Glyphosate Efficacy on Yellow Nutsedge (Cyperus esculentus)

Growers of glyphosate-resistant crops apply micronurrients tank-mixed with glyphosate to save time and production costs. Therefore, effect of zinc (Zn), as Zn sulfate, on absorption, translocation, and efficacy of glyphosare on yellow nutsedge was investigated. Glyphosate at 850 g ae ha(-1) provided 90\% yellow nutsedge control at 5 wk after treatment (WAT). Presence of Zn at 1,000 ppmw in the glyphosate spray solution reduced yellow nutsedge control to 24 and 8\%, 3 and 5 WAT, respectively. Yellow nutsedge control decreased with increasing Zn level (500 to 2,000 ppmw) in the spray solution. Yellow nutsedge treated with higher rates of Zn tank-mixed with glyphosate produced more tubers and tillers per plant than untreated plants. An abrupt decrease in absorption and translocation of (14)C-glyphosate occurred between 500 and 1,000 ppmw Zn. The antagonistic effect of Zn on glyphosate depended mainly on reduced absorption and translocation of (14)C-glyphosate within treated tissues. Less than 10\% of applied (14)C-glyphosate was absorbed when glyphosare was mixed with 1,000, 2,000, or 4,000 ppmw Zn as compared with 85\% absorption for glyphosate alone. These treatments inhibited > 90\% of (14)C-glyphosate translocation our of the treated leaf and > 50\% of tuber translocation relative to glyphosate alone. Results indicate that micronutrients containing Zn are not suitable for tank-mixing with glyphosate.