Optimizing metribuzin rates for herbicide-resistant <i>Amaranthus</i> weed control in soybean

Abstract Palmer amaranth and waterhemp are troublesome weeds in U.S. corn, soybean, and cotton production systems. Rapid evolution of resistance to herbicide from multiple sites of action in these species warrant alternate weed control options. Metribuzin applied preemergence can provide effective control of herbicide-resistant Amaranthus species. However, despite its decades of efficacy, many growers remain unaware of its weed control potential or are hesitant to use it due to concerns over crop injury. Field experiments were conducted in 2022 and 2023 in 15 states across the United States to investigate residual control of Palmer amaranth and waterhemp with metribuzin applied preemergence to soybean. Sites had either herbicide-resistant Palmer amaranth or waterhemp as the dominant weed species. Seventeen preemergence treatments were evaluated, including 13 doses of metribuzin (210 to 841 g ai ha -1 ), a dose of sulfentrazone (420 g ai ha -1 ), and a dose of S -metolachlor (1,790 g ai ha -1 ), along with nontreated and a weed-free control plots. Weed control and soybean injury were visually assessed and recorded at 14, 28, and 42 d after application (DAA) of preemergence herbicides. Additionally, weed density, weed biomass, and soybean height were recorded 28 DAA followed by a measure of soybean yield at maturity. Weed control was analyzed as a function of metribuzin dose and environmental factors using a generalized additive model. Crop injury of not more than 5% was predicted even with 841 g ai ha -1 of metribuzin. Metribuzin at 630 g ai ha -1 was more effective than sulfentrazone in delaying weed emergence and reducing weed density, while 315 g ai ha -1 of metribuzin outperformed S -metolachlor in both metrics. Metribuzin doses of 578 to 841 g ai ha -1 provided greater than 95%, 90%, and 80% weed control, respectively, at 14, 28, and 42 DAA. Higher metribuzin doses of 578 to 841 g ai ha -1 could be safely to effectively control herbicide-resistant Amaranthus weeds.