Over the last 20 years, herbicide resistant weeds have emerged as a major problem in many Illinois soybean fields. To date, nine different herbicide-resistant weed biotypes have been confirmed in the state. The herbicide resistance problem, however, has continued to spread across the state and possibly encompass other weed species.
With support from the Illinois Soybean Checkoff Board, researchers in the Department of Crop Sciences at the University of Illinois recently conducted a major survey of growers to better determine the exact scope of the problem and pinpoint important areas for future research.
"In our survey, 43 percent of the respondents reported they had encountered problems from herbicide-resistant weeds," says Christy Sprague, weed scientist with U of I Extension and coordinator for the weeds project. "They identified 26 different weed species with resistance to nine different herbicide classes. The earliest of these problems dates all the way back to 1986."
The survey indicated that waterhemp, ragweed, common lambsquarter, and common cocklebur comprised nearly 80 percent of the herbicide-resistant species in the state. Reports over the last few years have confirmed resistance in those four species to ALS inhibitors, triazine herbicides or even both of those classes.
"The respondents also identified some of those weeds as being resistant to other herbicides, including the PPO inhibitors, such as Flexstar, Ultra Blazer, and Cobra," Sprague says. "Since that survey, we have indeed confirmed some waterhemp with resistance to the PPO inhibitors."
U of I weed scientists Aaron Hager and Pat Tranel are now conducting additional research to pinpoint the exact mechanisms at work in this PPO resistance.
"The survey also included reports of waterhemp not being effectively controlled by glyphosate," Sprague says. "Although some of this may be due to environmental conditions, there are some populations that do not seem to fit that explanation. Although no waterhemp population in the state has been confirmed as resistant to glyphosate, there remains some concern that this could become a problem in the future."
She notes that researchers in several other Midwest states are working with some waterhemp populations that have not been effectively controlled with
glyphosate. They have determined that some of those populations show increased tolerance.
The Illinois survey also identified resistance problems in several other weed species that have not yet been confirmed as resistant in the state.
These include horseweed, velvetleaf, morningglory and woolly cupgrass.
"Most of these were reported by only a few respondents and may not truly be resistant," Sprague says. "In the case of horseweed, however, there are confirmed cases of resistance to ALS inhibitors in Ohio and glyphosate in Delaware, Tennessee, Kentucky, Ohio, Indiana, New Jersey, and Maryland. All
of which raises some concern that this could be a potential problem in Illinois."
According to Sprague, one major advantage of the survey is that it allows researchers to more easily identify what new weed species in the state may be developing herbicide resistance.
"The results let us get a better handle on what may be some of the emerging weed problems," she says. "We also can get a heads-up on the potential for certain weed species to develop resistance to major herbicides, such as glyphosate. With that kind of information, we can better focus our research on the areas that will be the most productive in dealing with any problem that we may confront in the future."