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Some Waterhemp has Developed Dicamba Resistance

by LuAnn Rolling, NRCS District Conservationist

According to a recent article in Successful Farming University of Illinois (U of I) weed scientists have confirmed resistance to the herbicide dicamba in a Champaign County (east-central Illinois) waterhemp population.

The population had never been sprayed with dicamba or its relative 2,4-D (both Group 4 synthetic auxin herbicides), to which it is also resistant.  According to the researchers waterhemp in the study population resists herbicides in five other site of action groups.

While that’s significant by itself – Dicamba has been widely adapted for use as a weed killer on crops with an engineered immunity – researchers say the more significant development might be the way in which this resistance was developed.  Pat Tranel, a professor in the Department of Crop Sciences at the University of Illinois, said, “This certainly shows that waterhemp has the ability to evolve resistance to this herbicide.”

If the waterhemp plants hadn’t seen Dicamba before, how did they know to resist it? While a lot of the specifics aren’t well understood, Tranel said the general mechanism involves the plant’s ability to handle other types of herbicides. The researchers found dicamba resistance was moderately heritable, meaning it could be passed on to offspring at least some of the time. Tranel says those genes are incompletely dominant, which explains why the team saw a range of responses from sensitive-like to up to tenfold resistant.

“We’re going to have to do something in addition to herbicides to try to get to the end of the growing season without any seed production,” he added. “Anything short of that and evolution continues.”

This topic was also noted in an article, “Characterization and inheritance of dicamba resistance in a multiple-resistant waterhemp (Amaranthus tuberculatus) population from Illinois,” published by Weed Science Society of America. Authors were Lucas Bobadilla, Darci Giacominni, Aaron Hager, and Pat Tranel. The work was funded by Bayer Crop Science.

The article stated, “We don’t understand this well enough to be able to predict what are the different herbicides that these metabolic resistances can confer resistance to. That’s the challenge for growers. So in the past, when we had resistance — typically we would have resistance to herbicides within the same site of action — we could tell farmers to rotate herbicides using different sites of action. But with this new type of resistance where it’s less predictable, we really can’t tell farmers which herbicides they should rotate.”

“If you extrapolate this, what this means is these populations are evolving resistance to herbicides that haven’t even been commercialized yet,” Tranel said. “So that’s kind of a scary thought, right? It’s harder and harder for companies to come up with new herbicides. And now you add on top of that, they may come up with a new herbicide and lo and behold, the weed population is already resistant to it.”

The development underscores the need for producers to diversify weed attack strategies, which could include crop rotations such as a small grain followed by a diverse cover crop that would allow for maximum weed suppression and possible grazing.