Waterhemp continues to “stack” different herbicide-resistant genes, including a biotype in western Illinois confirmed resistant to four herbicide groups:  glyphosate, PS II, ALS and PPO inhibitors.

“It’s the latest in a long line of resistant weeds, and the odds are stacked against growers,” says Pat Tranel, professor of molecular weed science at the University of Illinois. “Waterhemp’s continuing evolution of herbicide resistance is seriously limiting effective postemergence options, especially in soybeans.”

Tranel conducted a multiple-resistance survey in 2010 to determine the likelihood that other currently available postemergence soybean herbicides will be effective on glyphosate-resistant waterhemp populations.

“We found that all glyphosate-resistant waterhemp populations also contain resistance to the ALS inhibitors,” Tranel says. “But what’s surprising is that 40% of the glyphosate-resistant populations also contained resistance to PPO inhibitors.”

Their research also showed that the multiple resistance also exists at the individual plant level, which is a bigger problem. “A tankmix or sequential applications of glyphosate and a PPO inhibitor has the potential to control a multiple-resistant population, but not a multiple-resistant individual,” Tranel says.

Tranel’s recommendations, which he likes to call yield-protection management strategies, include rotating herbicide modes of action, using pre-emergence residual herbicides, scouting fields and dealing immediately with escapes and timely planting to get a good crop canopy.

“Weed resistance management is not a crop-specific problem, it’s an acre-specific problem,” says Chuck Foresman, manager of weed resistance strategies for Syngenta. “If you lose control of waterhemp in a field one year, you’ll likely have problems lurking in that same field for the next several, no matter what crop you rotate to.”

However, the problem did escalate last year in a central-Illinois field that was in seed corn production for seven consecutive years. According to Foresman, there was over-reliance on HPPD-inhibiting herbicides for broadleaf weed control. The first weed, a waterhemp biotype, to evolve resistance to this class of herbicides was discovered and later confirmed by University of Illinois researchers.

Research is ongoing to study how waterhemp populations evolve resistance, in the hopes of offering growers practical management solutions and preventing losses.

Based on a survey of retailers and consultants last year, Foresman estimates an average 5.5% yield reduction and an additional $16.90/acre in herbicide costs when resistance hits. “That’s not a one-year situation, weeds remember they are resistant,” he says.

Strategies should include using pre-emergence residual herbicides to control weeds as they germinate, Foresman says. They are more vulnerable because the seeds are exposed to the herbicide for a long time since it’s in the soil, and the plant’s underdeveloped enzyme systems render it more susceptible, he says.

“Using full-label rates and two overlapping modes of action to target a particular weed are also critical practices. Waterhemp has a very broad genetic base and has evolved to adapt to the way we farm. It comes in and takes over, especially in wet areas, and we have to pay attention,” Foresman says.

Chuck Schirz was paying attention when resistant waterhemp showed up in his 160-acre low-lying field just 30 miles from the Illinois River in Greene County, IL, in 2009. “It rained that morning, and our custom applicator sprayed glyphosate at noon,” he says. “We never got a handle on it the rest of the season, and it really hurt our yields.”

Schirz came back in 2010 at the advice of his Syngenta representative with a pre-emergence residual herbicide to clean up the field. “I will continue to use pre-emergent herbicides, full-label rates of all products and scout fields to help avoid problems in the future,” he says.