Waterhemp began emerging in the Fargo area in mid-May, indicating eastern North Dakota farmers are in for another year of battling the challenging weed and keeping their fields clean.
Last year, waterhemp resistance was confirmed to PPO-inhibiting (Group 14) herbicides in seven different counties in North Dakota. This winter, greenhouse testing also confirmed Group 14-resistant waterhemp in an additional county (Benson).
But the bigger news, according to Joe Ikley, NDSU Extension weed specialist, is that testing has also confirmed the first known waterhemp resistance to dicamba in North Dakota. A full dose response on one population from Traill County confirmed resistance. Samples from Griggs and Ramsey counties survived 3x rates of dicamba, and had a similar phenotypic response to the Traill County population.
“This population we’ve been working with is the first cast in our state that we’ve confirmed resistance,” Ikley said. “Back in 2019, we received some complaints about dicamba performance on waterhemp in southeastern North Dakota, but we couldn’t confirm it, but this past year, we had a population where we could. So for about four years we’ve had concerns, but as of the 2022 field season, we have dicamba resistance we have to contend with.”
Waterhemp began showing up in the southeastern part of the Red River Valley in the early 1990s, and over the last decade the weed has continued increasing in prevalence.
“To date, we can find waterhemp in the eastern half of North Dakota, mostly in the southeastern corner, and now it’s getting farther north into the Red River Valley where waterhemp is becoming, if not the worst, one of the top three worst weeds people have to deal with, and a lot of it is because of herbicide resistance,” Ikley explained.
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Waterhemp has already shown resistance to glyphosate and Group 2 herbicides, and more recently resistance has been confirmed with Group 14 herbicides and now dicamba.
“It’s a weed that produces a lot of seed, and with the issues we’re seeing with herbicide resistance, it’s becoming more challenging and difficult in crops like soybeans, dry edible beans, and sugarbeets, to keep those fields free of waterhemp throughout the growing season,” Ikley said.
The confirmation of dicamba resistance presents a big challenge to growers who utilize the XtendFlex soybean platform, as one of the reasons farmers grow that platform is for using dicamba.
“It’s a big deal because we’ve already taken away glyphosate, so we’re presumably relying on glufosinate (Liberty) and that type of soybean system. We could rotate to Enlist, because it’s still effective, but it’s going to spill over to other crops, as well,” Ikley said.
“For corn, we like to use Status, which is dicamba. We use it as a post-emergence to clean up any broadleaf weeds, but now that won’t be an option either, so some of these other options that we maybe don’t rely on but utilize on other crops that have dicamba, that will be problematic in those crops, as well,” he added.
As farmers are working hard to get their crops planted this spring as fast as they can, Ikley stresses the importance of spraying weeds at the appropriate size to avoid resistance.
“We’re facing another short planting season, and we’re warm with plenty of moisture right now, so weeds are growing fast. One of the best ways we can prevent herbicide resistance is spraying weeds at the appropriate size, which is quite small for most products. We like to target that 3-inch weed size or less – from my seat, 2 inches is preferable. If we can get out there with our post-emergence when they’re small, we can get good control and it will help us stay ahead of any resistance developing,” he concluded.