Waterhemp Claims Another Herbicide Group

The monster of a weed defies odds, but it isn’t running rampant yet ( Sonja Begemann )

Farmers across the Midwest know all too well the pervasiveness of waterhemp. One weed turns into two, and eventually catapults into a dominating force if left unchecked in fields.

This week, University of Illinois weed specialists found waterhemp has overcome yet another herbicide group: Group 15, which includes encapsulated acetochlor, S-metolachlor, metolachlor and dimethenamid. This marks the seventh mode of action to show failure against the weed.

The worst performing active ingredients within group 15 provided less than 25% control 28 days after application and less than 6% control at the 42-day mark. Researchers found resistance was present despite increased rates of product used, too.

“We found we could apply significantly higher than the labeled dose and still see resistance,” said Aaron Hager, University of Illinois Extension weed scientist, in a recent news release. He added that one particular active ingredient, S-metolachlor, only provided 10% control at the standard rate, 20% at twice the standard label rate and 45% at four times the label rate.

However, farmers likely won’t necessarily notice potential failure from group 15 herbicides as many are soil-applied, pre-emergent herbicides. As farmers know waterhemp germinates continuously throughout the season, so it’s hard to distinguish when the plant emerged and if it would have been exposed to soil-applied herbicides.

Because it might be difficult to know when a weed emerged, understanding resistance in individual fields could be a challenge. If farmers see weeds growing before they think a pre-emergent herbicide has ‘run out,’ they should reach out to an agronomist or Extension representative to confirm.

This particular resistance could be more unpredictable. Hager said he suspects metabolic resistance is the cause for group 15 failures—where the plant breaks down the herbicide before it can cause damage.

“As we get into the era of metabolic resistance, our predictability is virtually zero,” he added. “We have no idea what these populations are resistant to until we get them under controlled conditions. It’s just another example of how we need a more integrated system, rather than relying on chemistry alone.

“We can still use the chemistry but have to do something in addition. We want farmers to understand that we have to rethink how we manage waterhemp long-term,” Hager said.

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