Chris Peters and his father Greg work hard to keep their acres free of the resistant waterhemp escapes that dot nearby soybean fields where they farm in southeastern Nebraska.
“We try to mix up modes of action with our herbicides, but it can get confusing,” Chris noted. “Different names don’t necessarily mean different modes of action.”
Farmers like the Peters face a mind-boggling list of labels and names when it comes time to select herbicides. Each herbicide product has a brand name, an active ingredient name, and membership within a specific chemical family. Herbicides are also categorized by a group number that identifies their “site of action.” This number indicates the weed’s biochemical site targeted by the herbicide.
(The term “site of action” is often used interchangeably with “mode of action” within the industry, although the terms can have slightly different meanings in some contexts. “Mode of action” is a more general term that can refer to how the plant absorbs, metabolizes, and expresses injury from the herbicide. “Site of action” more specifically identifies the area of a weed affected by an herbicide.)
Using products with the same site of action over and over selects for resistance in the target weed. To avoid that, growers need to rotate between products that contain different and sometimes multiple sites of action each year.
Fortunately, farmers now have access to a handy sheet developed by the United Soybean Board, as part of “Take Action on Weeds,” an industry-wide effort designed to help prevent the spread of herbicide-resistant weeds.
This helpful guide to herbicide sites of action can be found at the University of Missouri’s weed science website or at the Take Action on Weeds website.
“You should put this wherever in your barn you’re mixing herbicides or by your desk in your office,” University of Missouri weed scientist Kevin Bradley told farmers at a university field day in northwest Missouri last week. “I’ve handed out thousands of these.”
One half of the guide organizes weeds by their site of action and the other side breaks down premix herbicides on the market into each of their active ingredients and their individual trade names and sites of action. Here’s more good news — almost every herbicide label now has the Group numbers listed to help you quickly identify what’s inside.
If you tend to lose papers as quickly as you get them, or combing through the long list of names hurts your eyes, you can try the Take Action on Weed’s mobile site of action lookup tool here. Simply enter the trade name or active ingredient of last year’s herbicides and the tool brings up a list of products with the same sites of action to avoid for this year’s line-up.
Whichever tool or chart you choose to use, do it now, because there is no one-stop resistant weed solution in our future, Bradley pointed out. “A lot of you might be thinking in the back of your mind I can wait until that super-duper new herbicide soybean trait comes out,” he said. “We have been trained to think that way.”
The new herbicide-tolerant crops on the way might add a tool to your weed management kit, but it’s not a new tool, he added. “Our last new mode of action was 30 years ago,” he said. “We’ve got to use multiple modes of action now.”