Yellow and knotroot foxtail are two common weed species infesting turfgrass and pastures in the southeastern region of the United States. Yellow and knotroot foxtail share morphological similarities and are frequently misidentified by weed managers, thus leading to confusion in herbicide selection. Greenhouse research was conducted to evaluate the response of yellow and knotroot foxtail to several turfgrass herbicides: pinoxaden (35 and 70 g ai ha–¹), sethoxydim (316 and 520 g ai ha–¹), thiencarbazone + dicamba + iodosulfuron (230 g ai ha–¹), nicosulfuron + rimsulfuron (562.8 g ai ha–¹), metribuzin (395 g ha–¹), sulfentrazone (330 g ai ha–¹), sulfentrazone + imazethapyr (504 g ai ha–¹), and imazaquin (550 g ai ha–¹). All treatments controlled yellow foxtail >87% with more than 90% reduction of the biomass. By comparison, only sulfentrazone alone controlled knotroot foxtail 90% and completely reduced aboveground biomass. Sethoxydim (520 g ai ha–¹), metribuzin, and imazaquin controlled knotroot foxtail >70% at 28 d after application. In a rate response evaluation, nonlinear regression showed that yellow foxtail was approximately 8 times more susceptible to pinoxaden and 2 times more susceptible to sethoxydim than knotroot foxtail based on log (WR₅₀) values, which were 50% reduction in fresh weight. Our research indicates that knotroot foxtail is more difficult to control across a range of herbicides, making differentiation of these two species important before herbicides are applied.

Nomenclature: dicamba; imazaquin; imazethapyr; iodosulfuron; knotroot foxtail; metribuzin; nicosulfuron; Pinoxaden; rimsulfuron; Setaria parviflora (Poir.) Kerguélen; Setaria pumila (Poir.) Roem. & Schult.; sethoxydim; sulfentrazone; thiencarbazone; yellow foxtail