Groundbreaking research conducted at the University of Wisconsin-Madison between 2022 and 2023 has confirmed that waterhemp (Amaranthus tuberculatus) can develop resistance to Group 14 PPO-inhibiting herbicides through preemergence applications, not just postemergence use as previously thought. The findings, recently published in the journal Weed Science, raise significant concerns for Midwest growers who rely heavily on PPO herbicides for early-season weed control.

The study was led by Felipe de Andrade Faleco, former Ph.D. student, and conducted under the guidance of Rodrigo Werle, Ph.D., Associate Professor and Extension Cropping Systems Weed Scientist at UW-Madison. The project was sponsored by the Wisconsin Soybean Marketing Board in collaboration with Pat Tranel, Ph.D., a leading molecular weed science researcher at the University of Illinois Urbana-Champaign.

"Our research documented the first confirmed case of a waterhemp population in Wisconsin that can survive PPO-inhibiting herbicides applied preemergence," said Werle. "Until now, most reports of PPO resistance were linked to postemergence applications. This shows resistance can evolve much earlier, in the soil phase."

PPO inhibitors, such as sulfentrazone (Spartan), fomesafen (Flexstar), and flumioxazin (Valor), block an enzyme crucial for chlorophyll production in plants. They have been widely adopted as preemergence tools to combat waterhemp populations that have already developed resistance to other postemergence herbicide groups.

The research involved comparing suspected PPO-resistant [A92 (R)] and known susceptible [A66 (S)] waterhemp populations exposed to a range of herbicide rates from 0× to 8× the labeled dose. After 28 days, the resistant biotypes showed clear survival and regrowth under soil-applied PPO herbicides.

"Discovering resistance at the preemergence level is concerning," Werle said. "It further narrows the chemical options available for early-season waterhemp control, which is already one of the most difficult and economically damaging weeds in the U.S. Midwest."

He emphasized that the findings should serve as an early warning for growers, agronomists, and ag retailers. With PPO inhibitors being one of the last reliable tools in the preemergence herbicide toolbox, the emergence of resistance at this stage could significantly impact weed control programs, yields, and input costs.

To slow the spread of resistance, researchers recommend:

• Always applying full labeled rates.

• Tank-mixing PPO herbicides with other effective preemergence herbicide groups.

• Integrating non-chemical tactics, including crop rotation, cover cropping, and mechanical control.

"Reducing reliance on a single mode of action is critical to preserving herbicide performance," Werle said. "We must rethink weed management as a system, not a single-product solution."

This new insight shifts the scientific understanding of herbicide resistance and urges a more integrated, proactive approach to weed control. As resistance traits evolve under both soil and foliar selection pressure, the agricultural community must respond with diverse, resilient management strategies.