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Fight Resistant Weeds by Knowing Modes of Action

Fight Resistant Weeds by Knowing Modes of Action
Agronomist says it's important to apply at least two modes of action each season.

Even if you only attended one meeting over the past six months, you probably heard about numbering and knowing herbicide modes of action. Before you write it off as academic mumbo-jumbo, read further. You might decide it should become common knowledge for everyone on your farm.

After all you're likely fighting resistant weeds, from marestail to Palmer amaranth. You need to know numbers for modes of action to help put together a plan that knocks out weeds and reduces odds for more resistant weeds.

That was the message Dan Childs brought to farmers at recent field days sponsored by DeKalb and Asgrow Genetics, and Monsanto. Childs is a technical weed control rep for Monsanto.

Out of control! The only mode of action for Palmer amaranth this tall is handweeding. Try controlling it instead with a burndown to start clean, and residuals or post products with multiple modes of action.

"Why do we care about the mode of action of herbicides?" Childs asks. "We care because we want to make sure we apply different modes of action each year.

"We don't want to apply several herbicides only to find out all were from the same family and use the same mode of action to kill weeds.

"We can best keep weeds confined by using different modes of action. That's how we can prevent further weed resistance."

Related: The Future in Weed Control Will Be Talking 'Mode of Action'

Here's an example. Suppose you pick up 80 acres for next year. It has some Palmer amaranth. Childs suggests assuming it's resistant to glyphosate and ALS- inhibitors. The mode of action for glyphosate is a specific synthase inhibitor, or number 9. ALS herbicides are classified as number 2. Forty-five weeds are resistant to ALS herbicides in the U.S., and 14 are resistant to glyphosate.

If your burndown is Roundup plus 2,4-D, you're choosing modes of action from families 9 and 2, respectively. Glyphosate will control other weeds, but not Palmer amaranth. Most studies show 2,4-D is inconsistent if that weed is more than 4 inches tall. Assume the Palmer amaranth plants were taller than four inches.

The best advice would be to come next with residual herbicide applications at planting. If you choose Valor XLT, which also contains Classic, you're applying modes of action from classes 14 and 2. Classic from number 2 won't help control ALS-resistant plants, but will control other weeds. So far you've applied one mode of action that's consistently effective against Palmer amaranth: Valor from class 14.

If you come back with Cobra post, also from class 14, you still only applied one effective mode of action all season.

Childs says that if you apply higher rates of Roundup, 2,4-D and Valor XLT, then follow with Cobra (class 14) with Warrant from class 15, now you get two modes of action in one season. The goal should be knowing which modes of action are effective, and applying at least two effective modes of action per season.

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