The Environmental Protection Agency has proposed increased restrictions and lower application rates for atrazine. Corn growers need to have options for weed control and increased scrutiny of atrazine may limit effective photosystem II-inhibiting herbicides. One alternative is the premixture of amicarbazone and metribuzin. The atrazine label prohibits planting soybean until the following year, limiting producers to replanting corn or grain sorghum after a failed stand. Amicarbazone allows a four-month soybean rotation interval, potentially enabling planting of the crop the same season as failed corn. Therefore, research was conducted in 2023 and 2024 in Fayetteville, AR, to evaluate soybean tolerance to an amicarbazone and metribuzin premixture after a simulated failed corn stand. Amicarbazone was applied at 245, 490, and 735 g ai ha-1 alone and in combination with metribuzin at 140, 280, and 420 g ai ha-1. Soybean was planted following at least 1.3 cm of rainfall, (19-20 days after application). The label allows amicarbazone and metribuzin to be applied to corn at 336 and 190 g ha-1, respectively, on silt loam soil with organic matter at 1.5 to 2%. The combination of amicarbazone and metribuzin at 735 and 420 g ha-1, respectively, more than twice the labeled rate for corn, induced 61 to 91% soybean injury 14 days after emergence (DAE). When amicarbazone and metribuzin rates were reduced to 245 and 140 g ha-1, respectively, the injury was 4% in both years, 14 DAE. Yield reductions were only seen in treatments with amicarbazone at 735 g ha-1 applied alone or in combination with metribuzin at 420 g ha-1. Overall, crop response and yield reductions should be expected with an amicarbazone and metribuzin premixture at the highest rates used in this study. However, the label for the premixture will not allow these rates to be applied.

The widespread use of atrazine in corn since the 1960s has raised environmental concerns, such as ground and surface water contamination. The U.S. Environmental Protection Agency has proposed label restrictions on atrazine to address these concerns and requires applicators to achieve herbicide mitigation points before applying herbicides. One way to achieve mitigation points is to reduce the proportion of the field that is treated. Therefore, research was conducted in 2023 in Arkansas, Mississippi, Tennessee, North Carolina, Indiana, and Virginia, and in 2024 in Arkansas, Tennessee, and Indiana, to determine whether targeted applications can mitigate atrazine use in corn while maintaining weed control levels comparable to those achieved with broadcast applications. All plots, except the nontreated control, received paraquat and S-metolachlor immediately after planting in 2023, with amicarbazone and metribuzin added in 2024. Combinations of atrazine, glyphosate, and mesotrione were applied postemergence either broadcast, target-applied to emerged weeds, or a combination of broadcast and target-applied. Targeted applications of herbicides did not differ in weed control, including Palmer amaranth and morningglory species, compared to broadcast applications of the same active ingredients. No injury or differences in corn grain yield were observed. Targeted applications in 2023 covered 86% of the area, on average, while 52% of the area was sprayed on average in 2024. Differences in the area sprayed during the targeted application between years can be attributed to the reduced area of weed emergence from a more robust residual herbicide combination in 2024. Based on this research, targeted spray technology can reduce atrazine use in corn while providing weed control comparable to that achieved with broadcast applications.