Texas wheat producers have observed reduced efficacy and failure to control Italian ryegrass with registered rates of sulfonylurea herbicides that were previously effective. Growth chamber studies were conducted to quantify the sensitivity and distribution of Italian ryegrass ecotypes in Texas to triasulfuron and to determine alternative herbicide management options. Italian ryegrass seed samples were collected from over 40 wheat fields in 13 central and north Texas counties where declining Italian ryegrass control was reported by farmers following sulfonylurea herbicide application(s). Two-leaf Italian ryegrass was screened with an application of 150 g ai/ha triasulfuron, a rate five times the registered herbicide use rate. Sensitivity was determined by the response of an ecotype to that of a known susceptible population. Of the 48 Italian ryegrass ecotypes sampled, nine were comparable to susceptible standard, while the remaining 39 ecotypes were less sensitive to triasulfuron. Four of the least sensitive ecotypes to triasulfuron plus the susceptible standard were selected for a subsequent study. Diclofop, clodinafop, and metribuzin reduced fresh weights by at least 69, 71, and 62% across all ecotypes. No imazamox or triasulfuron treatment reduced fresh weights more than 60%.
Nomenclature: Clodinafop; diclofop; imazamox; metribuzin; triasulfuron; Italian ryegrass, Lolium multiflorum L. #3 LOLMU; wheat, Triticum aestivum L.
Additional index words: ALS inhibitor, cross-resistance, herbicide resistance, imidazolinone, resistance survey, sulfonylurea.
Abbreviations: ALS, acetolactate synthase; ACCase, acetyl-CoA carboxylase; DAT, days after treatment; LTSE, low triasulfuron-sensitivity ecotypes.
