We conducted a field experiment in 2007 and 2008 and repeated it in a separate field in 2008 and 2009 to test the effectiveness of two herbicides applied at two different times on weed control during switchgrass establishment. At 4 and 6 wk after switchgrass planting (WAP), sulfosulfuron was applied at 0.04 and 0.07 kg ai ha⁻¹ with nonionic surfactant and quinclorac was applied at 0.28, 0.42, and 0.56 kg ai ha⁻¹ with methylated seed oil. Herbicides applied at 4 WAP tended to be more effective than at 6 WAP. Sulfosulfuron provided greater control of smooth pigweed; however, quinclorac provided greater control of yellow foxtail, a grass weed that has traditionally been difficult to control with herbicides in switchgrass. Average yellow foxtail control was 73, 87, and 93% for quinclorac at 0.28, 0.42, and 0.56 kg ha⁻¹, respectively, compared to 62 and 60% for sulfosulfuron at 0.04 and 0.07 kg ha⁻¹, respectively. Switchgrass injury (chlorosis and height reduction relative to the untreated control) was observed, but most symptoms were not detectable by 8 wk after treatment (WAT) for most treatments. Plots that received quinclorac at 0.56 kg ha⁻¹ at 6 WAP tended to have relatively low weed biomass and high total aboveground yield in the establishment year and relatively high total aboveground yield in the year after establishment.

Nomenclature: Quinclorac; sulfosulfuron; smooth pigweed, Amaranthus hybridus L.; yellow foxtail, Setaria pumila (Poir.) Roemer & J.A. Schultes; switchgrass, Panicum virgatum L.