Herbicide-resistant waterhemp will challenge Midwestern soybean growers this year. The challenge will be compounded by the weed’s increased resistance to herbicides with multiple modes of action, according to University of Illinois weed scientist Aaron Hager. This will limit the efficacy of herbicides on waterhemp and greatly complicate weed management programs, especially if the resistance is at a plant level and not just a field level.

“Field-level multiple resistance is when resistance to herbicides from more than one site of action is present within the population growing in any particular field,” Hager says. “When multiple-herbicide resistance occurs at the plant level, individual plants demonstrate resistance to herbicides encompassing more than one site of action.”

When three types of resistance are present at the field level and individual plants are resistant to only one herbicide, a grower can use a three-way tank mixture of herbicides A, B, and C to control all the plants in the field.

But this strategy will not work when individual plants possess resistance to the three herbicides. Tank mixtures of herbicides A, B, and C will not control the individual plants.

Hager says multiple resistance often occurs at both field and plant levels in one field. Plants resistant to just one herbicide (or herbicide family) populate the field with others plants that are resistant to multiple herbicide families. Tankmixing two or more postemergence herbicides may not effectively control herbicide-resistant waterhemp under these conditions.

University of Illinois weed science specialists have screened waterhemp for resistance to glyphosate, PPO inhibitors and ALS inhibitors. Last year, they screened 408 plants from 97 different fields suspected of having glyphosate-resistant waterhemp. Of the 408 plants submitted, 13% were sensitive to all three herbicides. The rest of the plants were resistant to at least one herbicide (28% to glyphosate, 19% to ALS inhibitors, and 3% PPO inhibitors).

On a field basis, the percentages were different. Only 4% of the 97 fields sampled had no herbicide-resistant waterhemp. About 25% of the fields contained waterhemp that was resistant to PPO inhibitors, 84% contained waterhemp resistant to ALS inhibitors, and 66% contained glyphosate-resistant waterhemp.

“Keep in mind that these fields — and plants within fields — were not randomly selected but were sampled based on suspected resistance to glyphosate,” Hager says. He also notes that the field-level results are based on small samples (five or fewer plants per field). “It is likely that, had a larger number of samples been taken, we would have found more fields with resistance to ALS and PPO inhibitors,” he says.

Even with the small sample size, over 10% of the fields contained three different types of herbicide resistance. This greatly increases the difficulty of managing weed populations using only postemergent herbicides labeled for use in conventional or glyphosate-resistant soybean varieties. Results from this and previous surveys suggest that glyphosate-resistant waterhemp occurs in many fields across the southern two-thirds of Illinois.

This survey was only one project in the ongoing research portfolio on herbicide-resistant weeds in Illinois. Weed scientists at several Illinois universities are studying this phenomenon and working to develop viable solutions for Illinois farmers. Much of this research is made possible by multiple public and private funding sources, including the Illinois Soybean Association.