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.

 

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