Response of Cotton at Different Growth Stages to Imazapyr

A significant proportion of the forested production area in South Carolina is managed using aerial applications of imazapyr. Cotton injury from off-target movement of imazapyr has been observed in South Carolina. Field experiments were conducted at the Edisto Research and Education Center (EREC) in 2021 and 2022 and at the Pee Dee Research and Education Center (PDREC) in 2022 to evaluate the response of cotton at two growth stages to imazapyr at 0.1, 0.05, 0.025, 0.0125, and 0.00625X of the normal use rate of 0.84 kg ae ha-1. Vegetative cotton injury was 86 and 74% at 0.1 and 0.05X imazapyr rates 28 days after application (DAA). Cotton heights ranged from 23 to 93 cm across all three locations (EREC 2021, EREC 2022, and PDREC 2022). Yields at EREC 2021 were reduced by 79, 48 and 31% at the 0.1, 0.05, and 0.025X rates, respectively. Similar reductions from imazapyr were observed at EREC 2022 and PDREC 2022. Reproductive cotton visual injury 28 DAA ranged from 95 to 64% for the 0.1 to 0.0125X rates, respectively. Reproductive cotton height reductions were 59% of the untreated control 28 DAA at 0.1X rate. Seed cotton yields ranged from 0 to 2880 kg ha-1 across three locations in 2021 and 2022. Seed cotton yield was lowest at the 0.1 to 0.025X imazapyr rates. Cotton exposure to imazapyr at the vegetative and reproductive growth stages resulted in plant injury, height, and yield reductions, especially at the higher rates of imazapyr. The highest reduction in cotton growth and yield was observed after exposure at the reproductive growth stage across the imazapyr rates. In summary, the magnitude of cotton response to imazapyr depends on crop growth stage and imazapyr concentration at the time of exposure with the greatest impact occurring at the reproductive growth stage.