Suspected Dicamba-Resistant Waterhemp

Two Potentially Dicamba-Resistant Waterhemp Populations Found in Iowa

Jason Jenkins
By  Jason Jenkins , DTN Crops Editor
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Waterhemp is one tough weed. Populations likely to be resistant to dicamba herbicide have been found in fields in central and east-central Iowa. Illinois and Tennessee already have confirmed resistance. (DTN photo by Pamela Smith)

JEFFERSON CITY, Mo. (DTN) -- Add Iowa to the list of states where populations of waterhemp resistant to the herbicide dicamba now reside.

In a Sept. 5 post on the Iowa State University Integrated Crop Management News website, ISU Extension field agronomist Meaghan Anderson reported the university had been notified by Bayer on Sept. 1 that suspected dicamba-resistant waterhemp populations had been sampled in two Iowa counties through extensive screening.

"Further testing and analysis will be required before Bayer can definitively state whether these specific waterhemp populations are resistant to dicamba," wrote a Bayer spokesperson in response to an inquiry from DTN. "Follow-ups are being conducted with the two growers who farm the specific fields where the populations exist."

In an email to DTN, Anderson said it was her understanding that the fields -- sampled in Scott County in 2021 and Marshall County in 2022 -- were discovered due to non-performance of Bayer's XtendiMax product, which is labeled for use in dicamba-tolerant soybeans. She noted that at this time, with the waterhemp's resistance to dicamba only reported to be "likely," the extent and level of any resistance is yet to be determined. She was not aware if these waterhemp populations were resistant to other herbicides.

"I don't believe they've been tested yet, but I hope they will be in the future," she said. "I can confidently say those populations will be found to be resistant at least to Group 2 (ALS inhibitors) and will likely be found to be resistant to other herbicides as well."

With these latest cases, there is now documented resistance to six herbicide groups in Iowa waterhemp populations. They include Group 2, Group 4 (auxins, such as dicamba and 2,4-D), Group 5 (triazines), Group 9 (glyphosate), Group 14 (PPO inhibitors) and Group 27 (HPPD inhibitors). Iowa joins Illinois and Tennessee where university researchers have previously documented waterhemp populations resistant to dicamba. Read more here:….

In its statement to DTN, Bayer said it has a comprehensive process in place to identify, sample, test and report suspected resistance to dicamba.

"As required by regulations for confirmed or suspected herbicide resistance, we've submitted this data to the EPA," the statement continued. "Bayer will continue to inform and educate growers in these areas on how to manage resistant weed populations and recommend future weed management programs and practices to help delay development of resistant populations in other fields these growers farm. Bayer will also follow up with Iowa State Extension to help educate other growers in the state."

Anderson wrote that while the discovery of these herbicide-resistant waterhemp populations is not a cause for panic, "It is a reality check for farmers who rely heavily on Group 4 herbicides or any other individual herbicide group for waterhemp control."

She offered the following management recommendations to help growers reduce the spread of waterhemp seed this fall:

-- Remove seed-producing waterhemp plants by hand and dispose of them outside the crop field. While this is a labor-intensive option, it would be very effective at reducing seed inputs in fields.

-- Avoid harvesting through drowned-out spots or other heavily weed-infested areas in fields.

-- Equip harvest equipment with harvest weed seed control tools.

-- Prioritize fields by waiting to harvest the weediest ones last, thus reducing the weed seed spread from problem fields.

-- Implement combine clean-out practices between fields to reduce seed spread.

She added that incidences of non-performance of products and suspected herbicide-resistant weed populations should be reported to their respective manufacturers.

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Jason Jenkins