Switchgrass (Panicum virgatum L.) is a North American native warm-season perennial grass that is a cornerstone species in conservation systems and can also be utilized as a high-quality forage. ‘Tusca’ is a cultivar of lowland switchgrass selected from ‘Alamo’ (USDA NRCS, Knox City, TX) for resistance to the herbicide imazapic. This study was conducted to determine if selection in Tusca conferred cross-protection to other ALS-inhibiting herbicides. Five ALS-inhibiting herbicides, including imazapic (IPIC), imazamox (IMOX), imazapyr (IPYR), imazethapyr (ITHR), and metsulfuron methyl (MSUL), were tested on Tusca, Alamo, and wildtype johnsongrass (Sorghum halepense L. (Pers.)) at five rates (25, 50, 75, 100, and 125% of the recommended label rate) plus an untreated control, under laboratory (PRE) and greenhouse (POST) conditions. Alamo was used as a negative control and johnsongrass was used as a reference species to confirm efficacy of herbicide treatments. Six replications of 25 seed of both switchgrass cultivars and johnsongrass were screened for response to herbicide treatment at germination, as well as the 3-leaf stage. Mean germination percentage for untreated Tusca, Alamo, and johnsongrass were 71.5, 24.8, and 40.8, respectively. Compared to controls, mean germination percentage of Tusca remained \u3e 50% at all rates of ITHR and IMOX. Alamo exposed to ITHR had \u3c 25% germination and \u3c 50% for IMOX. While Tusca shows some improved resistance to IPIC at germination, greater resistance was found to ITHR, IPYR, and IMOX, whereas mean germination percentage of Alamo was significantly reduced by all treatments
