UC Davis

California water-seeded rice is an intensely managed monoculture system that heavily relies on chemical weed control methods. The reliance and overuse of herbicide applications for weed management in continuous rice has led to an increase in selective pressure on already highly competitive weed species therefore leading to wide spread herbicide resistance. The spread of herbicide-resistant weeds is considered a major problem for rice production in California and there is a great need for new herbicide modes of action in rice. Tetflupyrolimet is a new herbicide with a novel mode of action, dihydroorotate dehydrogenase (DHODH) inhibition, that has strong activity on grasses. Three field studies were conducted to 1) determine control of Echinochloa spp. (watergrass species) and Leptochloa fascicularis (bearded sprangletop) by tetflupyrolimet 2) characterize the effects of tetflupyrolimet combined with other herbicides on weed control and rice growth and yield and 3) study differential rice varietal response to tetflupyrolimet. In the first study, tetflupyrolimet was applied at two timings and three rates followed by carfentrazone, a sedge and broadleaf herbicide, to focus on the activity of tetflupyrolimet on grass weeds. In the second study, tetflupyrolimet was applied at two timings and two rates in combination with or followed by carfentrazone, clomazone, thiobencarb, propanil, triclopyr, bensulfuron, or benzobicyclon plus halosulfuron at their respective rates and application timings to determine if tetflupyrolimet interacts with commonly used grass, sedge, and broadleaf herbicides in California water-seeded rice. In the third study, tetflupyrolimet was applied at two timings and four rates on six different rice varieties followed by carfentrazone, a sedge and broadleaf herbicide that causes no crop injury, to focus on the rice response to tetflupyrolimet while keeping plots free of sedge and broadleaf weeds. Tetflupyrolimet provided ≥ 98 and 97% control of Echinochloa spp. and bearded sprangletop, respectively, throughout the season across all treatments, which was comparable if not better than the grower standard treatments tested in susceptible populations. All herbicide combinations including tetflupyrolimet consistently controlled Echinochloa spp., bearded sprangletop, ricefield bulrush, smallflower umbrellasedge, ducksalad, and redstem comparably to a grower standard treatment of benzobicyclon plus halosulfuron followed by propanil followed by propanil and triclopyr. Tetflupyrolimet did not cause visible rice injury or yield reduction in the short (‘CM-203’), medium (‘M-105,’ ‘M-206,’ ‘M-209,’ and ‘M-211’), and long (‘L-208’) grain varieties that were tested. Tetflupyrolimet when applied in combination with commonly used sedge and broadleaf herbicides in a water-seeded system could be a very useful weed management tool for California rice growers.