Published online by Cambridge University Press: 20 January 2017
A yellow nutsedge biotype (Res) from an Arkansas rice fieldhas evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides.The Res biotype previously exhibited cross-resistance toALS inhibitors from four chemical families (imidazolinone, pyrimidinylbenzoate, sulfonylurea, and triazolopyrimidine). Experiments were conductedto evaluate alternative herbicides (i.e., glyphosate, bentazon, propanil,quinclorac, and 2,4-D) currently labeled in Arkansas rice–soybean productionsystems. Based on the percentage of aboveground dry weight reduction,control of the yellow nutsedge biotypes with the labeled rate of bentazon,propanil, quinclorac, and 2,4-D was < 44%. Glyphosate (867 g ae ha−1) resulted in 68 and > 94% control of the Res and susceptible yellow nutsedge biotypes,respectively, at 28 d after treatment. Dose-response studies were conductedto estimate the efficacy of glyphosate on the Res biotype,three susceptible yellow nutsedge biotypes, and purple nutsedge. Based onthe dry weights, the Res biotype was ≥ 5- and ≥ 1.3-foldless responsive to glyphosate compared to the susceptible biotypes andpurple nutsedge, respectively. Differences in absorption and translocationof radiolabeled glyphosate were observed among the yellow nutsedge biotypesand purple nutsedge. The susceptible biotype had less14C-glyphosate radioactivity in the tissues above the treatedleaf and greater radioactivity in tissues below the treated leaf compared tothe Res biotype and purple nutsedge. Reduced translocationof glyphosate in tissues below the treated leaf of the Resbiotype could be a reason for the lower glyphosate efficacy in the Res biotype. No amino acid substitution that wouldcorrespond to glyphosate resistance was found in the5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene of the Res biotype. However, an amino acid (serine) additionwas detected in the EPSPS gene of the Res biotype; albeit,it is not believed that this addition contributes to lower efficacy ofglyphosate in this biotype.
Associate Editor for this paper: Timothy Grey, University of Georgia.