A yellow nutsedge biotype (
Res) from an Arkansas rice field has evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides. The Resbiotype previously exhibited cross-resistance to ALS inhibitors from four chemical families (imidazolinone, pyrimidinyl benzoate, sulfonylurea, and triazolopyrimidine). Experiments were conducted to evaluate alternative herbicides (i.e., glyphosate, bentazon, propanil, quinclorac, and 2,4-D) currently labeled in Arkansas ricesoybean production systems. 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 ha1) resulted in 68 and > 94% control of the Resand susceptible yellow nutsedge biotypes, respectively, at 28 d after treatment. Dose-response studies were conducted to estimate the efficacy of glyphosate on the Resbiotype, three susceptible yellow nutsedge biotypes, and purple nutsedge. Based on the dry weights, the Resbiotype was 5- and 1.3-fold less responsive to glyphosate compared to the susceptible biotypes and purple nutsedge, respectively. Differences in absorption and translocation of radiolabeled glyphosate were observed among the yellow nutsedge biotypes and purple nutsedge. The susceptible biotype had less14C-glyphosate radioactivity in the tissues above the treated leaf and greater radioactivity in tissues below the treated leaf compared to the Resbiotype and purple nutsedge. Reduced translocation of glyphosate in tissues below the treated leaf of the Resbiotype could be a reason for the lower glyphosate efficacy in the Resbiotype. No amino acid substitution that would correspond to glyphosate resistance was found in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene of the Resbiotype. However, an amino acid (serine) addition was detected in the EPSPS gene of the Resbiotype; albeit, it is not believed that this addition contributes to lower efficacy of glyphosate in this biotype.