Waterhemp resistant to dicamba herbicide has now been confirmed in Missouri, further solidifying concerns over growing herbicide resistance in key Midwestern and Southern cropping systems. The announcement, made this week at the University of Missouri Crop Management Conference, marks Missouri as the fourth state to officially detect dicamba-resistant waterhemp, joining Illinois, Tennessee, and Iowa.

The resistant population was collected in 2023 from a field in Saline County, located in west-central Missouri, and confirmed through greenhouse trials conducted by MU Extension weed scientist Kevin Bradley and his lab team. "We've been looking at it in the greenhouse since May," Bradley told DTN. "We did a couple runs, and we feel like it's dicamba resistant." The affected plants could not be controlled with a labeled rate of dicamba and showed significant differential response when compared to known susceptible populations.

While the announcement formalizes what many agronomists have suspected for some time, Bradley acknowledged that "dicamba resistance in waterhemp has likely been in Missouri already-it just hadn't been confirmed." The identified population remains susceptible to glufosinate and 2,4-D, though experts warn that resistance stacking remains a looming threat.

This confirmation comes at a critical regulatory moment. The EPA is currently reviewing public comments on its proposed unconditional reregistration of three "over-the-top" (OTT) dicamba herbicides: Engenia (BASF), XtendiMax (Bayer), and Tavium (Syngenta). These products were conditionally registered in 2020, but a federal court ruling in 2024 vacated those approvals due to FIFRA violations, leaving U.S. soybean and cotton growers without OTT dicamba options for the 2025 season.

With EPA deliberating whether to allow these products back on the market in 2026, Bradley emphasized the urgency of diversified weed management. "If dicamba returns, we can expect even more resistance unless farmers start relying less on single postemergence herbicides," he told attendees.

Bradley stressed the importance of integrated pest management (IPM) and multiple effective modes of action, particularly through preemergence herbicides, to delay resistance development. "I really encourage you to use two or three effective modes of action on our preemergence herbicides," he said. "Whether it's Palmer amaranth down south or waterhemp in the Midwest, we now have resistance to nearly every herbicide class."

His broader warning resonates across U.S. row crop systems: "If it can happen in one place, it can happen anywhere." The Missouri finding serves as yet another wake-up call for soybean and cotton producers to reevaluate their long-term weed control strategies in an era of mounting resistance.