This photo shows the results of Felipe Faleco’s greenhouse experiment analyzing how different rates of Spartan 4F (sulfentrazone), Flexstar (fomesafen) and Valor EZ (flumioxazin) affected waterhemp growth when applied at pre-emergence. There were escapes with all 3 herbicides applied at the label rate, especially Flexstar. Photo by: Felipe Faleco

Label Rates Not Enough to Control Herbicide-Resistant Waterhemp

Greenhouse trial shows Flexstar particularly ineffective with 4 times the label rate needed to control waterhemp

Label-rate applications of PPO herbicides can’t control herbicide-resistant waterhemp, according to greenhouse trials conducted by University of Wisconsin weed scientists

In 2021, a Dodge County, Wis., crop consultant identified PPO-resistant waterhemp and asked Rodrigo Werle’s University of Wisconsin-Madison research lab to investigate.

“Year after year of using sulfentrazone in pre-emergence, he started to observe some escapes,” says Felipe Faleco, a graduate student at Werle’s lab. “Sulfentrazone was not enough anymore to control this population. He collected seeds for this population and sent them to us, and we started an investigation in the greenhouse.”

Pre Concerns

Faleco designed pre-emergence dose response experiments to assess the waterhemp’s level of resistance. The pre-dose resistance experiment applied different rates of Spartan 4F (sulfentrazone), Flexstar (fomesafen) and Valor EZ (flumioxazin). Herbicide rates ranged from 0.015 to 8 times the label rate, which was 8 fluid ounces per acre for Spartan, 1 pint per acre of Flexstar and 3 fluid ounces per acre of Valor EZ. 

“We used rates below and above the label rate to try to identify if it’s low-level or high-level resistance,” Faleco says. 

“Residual herbicides are not lasting as long as they should…”

There were escapes with all 3 herbicides applied at the label rate, especially Flexstar. The Flexstar trial wasn’t completely clean until the herbicide was applied at 4 times the label rate. 

“PPO herbicides are our foundation for residual control,” Werle says. “What Felipe is documenting here in PPO resistant waterhemp is that the residual herbicides are not lasting as…

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Michaela paukner

Michaela Paukner

Michaela Paukner is the managing editor of No-Till Farmer, Strip-Till Farmer and Cover Crop Strategies. Her previous journalism experience includes working as a reporter for a legal magazine and as a producer for two Wisconsin TV news stations. She has also worked with clients across the globe as a freelance writer and marketing consultant, and as a brand manager for a Wisconsin-based boutique marketing agency. She's a graduate of the University of Wisconsin-Green Bay.

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