Safety of Recognition®-based tank-mixes with fluazifop-P-butyl on selected St. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze] cultivars in Florida

Abstract

Recognition^® is a new trifloxysulfuron-sodium formulation containing metcamifen safener (i.e., trifloxysulfuron-sodium & metcamifen; TSM) enabling tank-mixing with fluazifop-P-butyl (FPB) for selective bermudagrass (Cynodon spp.) control in St. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze], the predominant lawn species in Florida. Three studies were conducted from August to November of 2022 and May to October of 2023 in north-central Florida to ensure the safety of TSM ± FPB to CitraBlue, Classic, Floratam, Palmetto, and Seville St. Augustinegrass maintained as a home lawn. Herbicides (TSM at 18 or 28 g ai ha⁻¹ and FPB at 210, 315, or 420 g ai ha⁻¹ alone or combined) were applied two or three times every 4 weeks from August 12, 2022 (Study 1), twice every 6 weeks from June 7, 2023 (Study 2), and once or twice every 6 weeks beginning in the spring (from May 5) or in the summer (from July 11) of 2023 (Study 3). All treatments in Study 1 provided excellent bermudagrass control but caused severe injury to Seville St. Augustinegrass. In Study 2, only FPB alone led to unacceptable damage, with no differences in herbicide tolerance among CitraBlue, Floratam, and Palmetto cultivars. Study 3 confirmed CitraBlue safety to TSM-based treatments and a higher risk of injury in Seville. Overall, greater phytotoxicity was observed with later-season applications (summer in Study 3 and late summer in Study 1), suggesting increased sensitivity in more susceptible St. Augustinegrass cultivars as the season progresses. While the exact reasons for this require further investigation, our results suggest that metcamifen may not provide effective safening in certain sensitive cultivars, particularly with late-season applications. Therefore, practitioners should exercise caution when using TSM, ideally conduct small-scale tests before broadcasting, and avoid applications in late summer or fall.