Waterhemp has done it again. University of Illinois (U of I) researchers have published an article in Pest Management Science confirming that waterhemp is the first weed to evolve resistance to HPPD-inhibiting herbicides.

"A fifth example of resistance in one weed species is overwhelming evidence that resistance to virtually any herbicide used extensively on this species is possible," says Aaron Hager, U of I Extension weed specialist.

Waterhemp is not a weed species that can be adequately managed with one or two different herbicides, Hager says. This troublesome weed requires a much more integrated approach.

"Large-scale agronomic crop production systems currently depend on herbicides for weed management," Hager says. "A weakness in this approach lies in its strength; because herbicides are so effective, they exert tremendous selection pressures that, over time, result in resistant weed populations as natural outcomes of the evolutionary process."

In an article in the Journal of Agricultural and Food Chemistry, Hager and Pat Tranel, a U of I professor of molecular weed science in the Department of Crop Sciences, shared the results of a survey of multiple-herbicide resistance in waterhemp. The results showed that all populations resistant to glyphosate were also resistant to ALS inhibitors and 40% contained resistance to PPO inhibitors.

Adding HPPD resistance to the mix complicates problems for growers and scientists. When weeds stack several forms of resistance, it greatly reduces the number of viable herbicide options.

"We are running out of options," Hager says. "This multiple-herbicide resistance in waterhemp has the potential to become an unmanageable problem with currently available postemergence herbicides used in conventional or glyphosate-resistant soybean."

Hager says they've already discovered one waterhemp biotype that's resistant to four different herbicide families. He says growers may see five-way resistance in the future.

Fortunately, there are very few annual weed species in the U.S. that have shown this level of multiple resistance. Waterhemp is a dioecious species and ideally suited for evolving herbicide resistance by sharing resistance genes among populations and biotypes.

"For example, you can have HPPD resistance evolving in field A, and in adjacent field B you can have selection for glyphosate resistance," Tranel says. "Pollen is always moving in the air, allowing pollen from field A to mix with resistant plants from field B resulting in HPPD and glyphosate resistance in the same progeny. That's how easy it is to stack resistance."