URBANA, Ill. – Waterhemp resistance to dicamba in Champaign County is showing 5-to 10-fold levels compared to susceptible populations, according to University of Illinois Department of Crop Sciences weed experts.

Overall, the U of I weed population is showing 6-way resistance to herbicides classified as Group 4 (dicamba and 2, 4-D), Group 2 (ALS-inhibitors), Group 5 (triazines), Group 14 (PPO-inhibitors), Group 27 (HPPD-inhibitors) and Group 15 (VLCFA-synthesis inhibitors), DTN reported last week.

The U of I researchers are concerned about the level of resistance since the population in question was not treated extensively with dicamba.

Pat Tranel, a University of Illinois molecular weed biologist, said research suggests it involves a type of metabolic resistance, which allows weeds to escape herbicide damage by rapid metabolism of a chemical. This allows weeds to survive multiple classes of herbicides, including groups they have not been exposed to. There is also evidence of cross-resistance between this weed’s multiple herbicide-resistance traits, the biologist noted.

“This kind of cross resistance really hampers us in our ability to tell farmers what to do,” Tranel said.

In addition, Larry Steckel, University of Tennessee weed specialist, reported waterhemp populations showing roughly 4.5-fold levels of resistance to dicamba after working with Purdue University to confirm his findings. He reported that the populations under study are also resistance to Group 9 (glyphosate), Group 14 (PPO-inhibitors) and Group 2 (ALS-inhibitors).

Steckel said that Tennessee farmers began ramping up their use of dicamba in 2016, after dicamba-tolerant (Xtend) crops became rapidly and uniformly accepted.

“We’ve been priming the pump. I think that’s why we’re seeing dicamba-resistant weeds in our state quicker than in others,” Steckel said, adding that a surprising discovery in the university’s field research was that though dicamba did not control the waterhemp, 2,4-D appeared to be very effective. “This was very different than what we have seen with dicamba-resistant Palmer amaranth in Tennessee as, to date, it has always been resistant to 2,4-D as well.” 

Bill Johnson, Purdue weed scientist, along with his research team, had been studying reported dicamba-resistant populations of waterhemp in one Tennessee soybean field since 2019. “When we were told that the waterhemp had survived multiple dicamba applications it really got our attention,” he said.

Johnson is actively screening Indiana waterhemp populations that show likely dicamba resistance, as are experts from the University of Missouri, DTN reported. UM weed scientists released a study earlier in 2021 showing declining sensitivity to dicamba and glufosinate among waterhemp populations in seven different states.

“It is what many of us predicted would happen because we’re not starting from scratch with dicamba” with Xtend crops, Hager said. “We’ve already had decades of selection against it, because we’ve used it for so long in agriculture.”

A 2019 study from the U of I Department of Crop Sciences reported by Farm World documented waterhemp’s emerging resistance to Group 15s. With its newfound resistance to Group 15 herbicides, waterhemp had evolved resistance to at least seven of the nine herbicide MOAs (modes of action) that can be used on the plant, according to department head Adam Davis.

“In some areas, we are one or two MOAs away from losing chemical control of waterhemp and other multiple-herbicide-resistant-weeds. And there are no new herbicide MOAs coming out. There haven’t been for 30 years,” said Davis, who, along with Hager, was co-author of the U of I study.