Rice performance following exposure to sub-lethal concentrations of paraquat at multiple growth stages

Abstract Rice in the midsouthern U.S. is often planted in the same timeframe as preplant or preemergence herbicide applications of paraquat are applied to other row crops in adjacent fields. Additionally, off-target movement of paraquat may occur on rice late in the season due to paraquat’s utilization as a harvest aid in soybean. Although previous research has documented rice response to early- and late-season exposure to sub-lethal concentrations of paraquat, research quantifying rice injury after multiple exposures is not available. Therefore, research was conducted from 2019 to 2021 at the Mississippi State University Delta Research and Extension Center in Stoneville, MS, to evaluate rice growth and yield after exposure to sub-lethal concentrations of paraquat multiple times during the rice life cycle. Paraquat was applied at 84 g ai ha -1 to rice in the spiking to one-leaf (VEPOST) and three- to four-leaf (MPOST) growth stages to simulate early-season exposure, while late-season exposure of paraquat was simulated with applications at 28 g ai ha -1 when 50% of rice plants in a plot had visible panicles (HEAD), to rice the day of draining (DRAIN), and a sequential treatment with rice exposure at HEAD followed by (fb) DRAIN. Rice injury was greatest pooled across early-season exposure, when paraquat exposure occurred at HEAD fb DRAIN. Rough rice grain yield was reduced 15 and 18% from HEAD and HEAD fb DRAIN exposures, respectively, compared with the no late-season exposure treatment. Whole milled rice yields were not impacted by multiple exposures to paraquat. Multiple exposures to paraquat had the greatest effect on rice performance when the exposure occurred in both early- and late-season.