As farmers across the Corn Belt were wrapping up the 2019 harvest, the buzz among corn and soybean producers centered on reports from Illinois that the tough-to-control weed, waterhemp, had shown resistance to yet another class of chemical weed control — those in Group 15.

“Herbicides are grouped into classes based on their mechanism of action (MOA), or the specific target within a plant that’s vulnerable to the compound’s attacks,” says Aaron Hager, University of Illinois weed scientist. “Because of various regulations and biological and agronomic realities, a limited number of herbicide MOAs can be used on any given crop and the suite of weeds that accompanies it.

“Historically, about nine herbicides have been useful for waterhemp — and now the weed appears to be resistant to at least seven of those,” he explains.

To date, waterhemp is documented resistant to the following herbicide classes:

• ALS inhibitors (Group 2)
• Synthetic auxins/growth regulators (Group 4)
• Photosystem II inhibitors (Group 5)
• EPSP synthase inhibitors (Group 9)
• PPO inhibitors (Group 14)
• Fatty acid synthesis inhibitors (Group 15)
• HPPD inhibitors (Group 27)

Hager’s associate in reporting the latest development, and head of Crop Sciences at Illinois, Adam Davis, says in some areas, growers are only one or two MOAs away from completely losing chemical control of waterhemp and other multiple-herbicide-resistant weeds. “There are no new herbicide MOAs coming out. There haven’t been for 30 years,” he adds.

“We don’t want to panic growers, but they need to…