Start counting. Glyphosate is now a Group 9 herbicide. Valor is Group 14. The new corn herbicide called Peak is a Group 2. The premix Capreno—it’s a Group 2 and Group 27. It’s all part of a new herbicide labeling system that groups products by site of action. Knowing the site of action is a key to developing a systems approach to managing weed resistance.
The Canadians know all about it. So do the Australians. Farmers in both of those countries have used a standardized numeric system to help them rotate herbicide chemistry for years. Peter Sikkema, a weed scientist at the University of Guelph, says farmers in Canada are much more likely to tell you they are struggling with Group 2 herbicide resistance than to mention ALS-resistance. "We adopted the system about 10 years ago," says Sikkema.
"The system is particularly helpful in sorting through premix combinations and breaking down trade names into activity," he adds. The group numbers are based on a system developed by the Weed Science Society of America (WSSA).
Growers have come to understand that rotating herbicide mode of action (MOA) is important in managing resistant weeds. However, recently scientists have deemed site of action—or where the herbicide actually works within the plant—to be even more specific. Repeated use of herbicides with the same site of action can result in the selection of herbicide-resistant weed populations.
Iowa State University weed scientist Bob Hartzler says the intent of the labeling is to simplify development of herbicide programs that reduce the likelihood of selecting herbicide resistant weeds. Generally, the greater the number of sites of action used, the less selection pressure placed on weeds.
"However, designing an integrated program is not as simple as randomly adding sites of action," says Hartzler. "You also need to be sure the herbicide being used has good activity on the important weeds in the field to successfully reduce selection pressure."
Here’s an example: Say you’ve long depended on a glyphosate herbicide program and as a result, have developed a glyphosate-resistant waterhemp problem. Simply adding a Group 2 herbicide would provide little benefit since most waterhemp is also resistant to these herbicides. Perhaps your problem is giant ragweed--a Group 15 herbicide would provide little benefit on that weed or other large-seeded broadleaves due to its poor activity on these weeds.
The new herbicide labeling remains voluntary in the United States, but the Environmental Protection Agency has requested that registrants add group numbers to labels. You can expect to see the codes pop up more frequently as the push continues to delay development of resistance. After all, Group 1 is much easier than saying acetyl CoA carboxylase or ACCase Inhibitor.
To better understand how and why WSSA develops site of action groups: