COLUMBIA, Mo. (DTN) -- Missouri has joined the list of states where at least one population of waterhemp has been declared resistant to dicamba herbicide.

During the University of Missouri Crop Management Conference this week, MU Extension weed scientist Kevin Bradley announced that his lab had recently determined that a population of the troublesome weed species, collected in 2023 from a field in Saline County in west-central Missouri, was resistant to dicamba. This means that the plants could not be killed with a labeled rate of the herbicide and showed a significant differential in response when compared to plants from a known susceptible population.

"I don't think this is any grand announcement because quite frankly, I think we've had dicamba resistance in waterhemp already in this state," he told the audience of farmers and crop advisers. "It just hadn't been confirmed."

With the confirmation from Bradley's lab, Missouri becomes the fourth state to officially identify a dicamba-resistant waterhemp population. Populations were documented in 2021 in Illinois and Tennessee and in 2023 in Iowa.

"We've been looking at it in the greenhouse since May," Bradley told DTN following the conference. "We did a couple runs, and we feel like it's dicamba resistant. It was still susceptible to glufosinate and 2,4-D."

The news of this latest dicamba-resistant weed population comes as EPA considers thousands of public comments generated by the agency's proposed decision this past summer to approve the unconditional reregistration of three "over-the-top" (OTT) dicamba products intended to control broadleaf weeds postemergence in dicamba-tolerant soybeans and cotton.

Those products -- including BASF's Engenia, Bayer's XtendiMax (referred to as KHNP0090) and Syngenta's Tavium -- had previously received conditional, five-year registrations in October 2020, but those labels were vacated in February 2024 by a federal court in Arizona that determined the agency had violated the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) by approving the registrations without providing notice and an opportunity for public comment. Farmers in 2024 were allowed to use the herbicides under an existing stock order, but OTT dicamba was unavailable to soybean and cotton farmers this past growing season.

While Bradley characterized the discovery of dicamba-resistant waterhemp in Missouri as "no big surprise," he told the conference attendees that it's relevant because in his estimation, there's strong likelihood that OTT dicamba products could be back on the market in 2026, providing opportunity for more resistance to develop.

The weed scientist urged the audience to look beyond postemergence herbicides and adopt integrated pest strategies and new technologies for weed control.

"I don't go out actively trying to find these resistant weeds, but when we find them, it's a good lesson to show growers that it's real and it can happen," he said. "I really encourage you to use two or three effective modes of action on our preemergence herbicides.

"Whether it's Palmer amaranth down south or waterhemp in the Midwest and beyond, we have resistance to just about every herbicide solution now," Bradley said. "If it can happen in one place, it can happen anywhere."

The Getting Rid of Weeds (GROW) network is a scientist-led organization helping farmers with a greater diversity of weed-control strategies to complement chemical use. Learn more here: https://growiwm.org/…

Jason Jenkins can be reached at jason.jenkins@dtn.com

Follow him on social platform X @JasonJenkinsDTN

(c) Copyright 2025 DTN, LLC. All rights reserved.