Two Likely Dicamba-Resistant Waterhemp Populations Found In Iowa
Another state, Iowa, likely now has populations of dicamba-resistant waterhemp.
HG 4-resistant waterhemp populations were previously documented by university researchers in three other states: Nebraska in 2009, Illinois in 2016, and Missouri in 2018.
In a notice distributed by Iowa State University Extension, it reports the two suspected populations in Iowa were initially tested by Bayer Crop Science in Scott County in 2021 and in Marshall County in 2022. The company says it will continue testing these populations to positively confirm resistance.
“This is the first case of plant growth regulator (HG 4) resistant waterhemp in Iowa, though other states have already documented HG 4 resistance in their own waterhemp populations,” notes Meaghan Anderson, a field agronomist in central Iowa and an Extension field specialist at Iowa State University Extension and Outreach.
An Important Reality Check
This discovery is not a cause for panic, Anderson adds. “But it is an important reality check for farmers who rely heavily on HG 4 herbicides or any other individual herbicide group for waterhemp control,” she says in an online article available here.
Waterhemp is known for its ability to quickly adapt to herbicide management tactics and is considered one of the most troublesome agronomist weeds in the Midwest, where it has rapidly evolved resistance to multiple herbicides, according to the Weed Science Society of America (WSSA).
In Illinois, the WSSA cited one waterhemp population in 2021 as having exhibited resistance to six herbicide groups – in addition to dicamba: 2,4-D, ALS inhibitors, PROTOX inhibitors, HPPD, PSII inhibitors and very long chain fatty acid synthesis inhibitors.
In Iowa, Anderson says that Iowa State University researchers have officially documented resistance to five HGs in Iowa waterhemp populations (see Table 1). 
In a University of Illinois College of Agricultural, Consumer and Environmental Sciences article authored by Lauren Quinn and available here, Aaron Hager, Extension weed scientist, emphasizes preventing or eliminating weed seed production.
“Look, herbicides will continue to be valuable tools to help prevent crop yield loss, but we also must consider additional tactics to ensure no waterhemp plant is allowed to produce seed during the growing season," Hager said. "All we know with certainty is that if plants can’t produce seed, the frequency of any resistance mechanism can’t change or grow.
"Even a few seed-bearing waterhemp plants remaining in the field at harvest could contain the newest herbicide-resistance mechanism,” Hager added. “It’s worth repeating: consider additional tactics to ensure no weed seed production.”
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