LibertyLink®, WideStrike® and XtendFlex® Tolerance to Late Postemergence Applications of Glufosinate and S-Metolachlor

Glufosinate remains an important postemergence (POST) herbicide for controlling glyphosate-resistant Amaranthus palmeri (S. Wats) in auxin-tolerant trait systems. Although visual injury from glufosinate applications to WideStrike cultivars is expected, concerns have been raised on the visual injury noted in XtendFlex cultivars, particularly with sequential late-POST glufosinate applications or when S-metolachlor is tank-mixed with glufosinate. Field trials were established in Jackson, TN during 2015 and 2017 and in Huntersville, TN during 2016. Herbicide treatments included an untreated; one, two, and three sequential applications of glufosinate; and glufosinate + S-metolachlor followed by (FB) glufosinate FB glufosinate + S-metolachlor. Applications began 40 days after planting and sequential applications were made every 10 to 14 days. Cultivars included DP 1522 B2XF (DeltaPine, Bayer CropScience, St. Louis, MO), PHY 333 WRF (Phytogen, Corteva Agriscience, Indianapolis, IN), and ST 4946 GLB2 (Stoneville, BASF Corp., Florham Park, NJ). Visual injury ratings varied across timing, treatment, cultivar, and site-year. Three sequential applications of glufosinate with two applications of S-metolachlor caused 7 to 20%, 2 to 15%, and 1 to 8% injury 10 days after the last application in PHY 333 WRF, DP 1522 B2XF, and ST 4946 GLB2, respectively. Cultivar lint yield and fiber quality did not vary by herbicide treatment. Producers who apply glufosinate should expect increasing visual injury from LibertyLink to XtendFlex to WideStrike cultivars, with a sharp increase in visual injury from XtendFlex to WideStrike cultivars; however, sequential, labeled applications of glufosinate with or without two applications of S-metolachlor will likely not impact yields of LibertyLink, WideStrike, or XtendFlex cultivars.