Influence of Simulated Rainfall Timing on the Efficacy of Hexazinone and Quinclorac for the Control of Knotroot Foxtail ( <i>Setaria parviflora</i> )

Abstract Knotroot foxtail is a troublesome perennial grass found in pastures across the Southeastern United States. Herbicides such as hexazinone and quinclorac are labeled for control of this weed; however, their efficacy can be inconsistent due to delayed or excessive rainfall, which limits herbicide movement into the soil for root uptake, allowing knotroot foxtail rhizomes to survive and produce new shoots, resulting in reduced control. A greenhouse study was conducted at Auburn University, Alabama, in 2023 and 2024 to evaluate the effect of rainfall timing on the efficacy of quinclorac and hexazinone in controlling knotroot foxtail. Knotroot foxtail plants averaging 28 cm in height were treated with quinclorac (0.4 kg ae ha -1 ) or hexazinone (0.8 kg ai ha -1 ), followed by simulated rainfall (6.3 mm) applied at 0, 3, 6, 9, 12, and 15 days after herbicide treatment. Hexazinone provided greater knotroot foxtail control and rhizome biomass reduction than quinclorac. At 51 days after each rainfall treatment (DAERT), hexazinone provided 90% control, compared with 76% with quinclorac. In 2024, at 51 DAERT, control with hexazinone ranged from 99% to 92% when rainfall occurred within 0 to 6 days after application, but declined to 85% and 81% when rainfall was delayed until 12 and 15 days, respectively. Similarly, quinclorac-treated plants achieved 87% to 77% control when rainfall occurred within 0 to 9 days, but control dropped to 67% to 62% with delayed rainfall at 12 to 15 days. Relative to the nontreated control, hexazinone and quinclorac reduced knotroot foxtail rhizome biomass by 72% and 42%, respectively. Early rainfall (0 to 6 days) after herbicide application enhanced knotroot foxtail control, while delayed rainfall reduced herbicide effectiveness. This study underscores the importance of the timing of application when hexazinone or quinclorac is used for knotroot foxtail management before precipitation events.