Know Your Corn Herbicide Numbers

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A Michigan State University Extension weed specialist breaks down how to choose the correct modes of action and sites of action to prevent herbicide resistance.

Optimum weed control, especially when targeting herbicide-resistant weeds, requires you to understand, select and rotate the right herbicide sites of action.

“We encourage growers to use the Take Action herbicide classification chart when you have a resistant weed issue,” says Christy Sprague, Michigan State University Extension weed specialist. “Using it in conjunction with the current weed control guide compiled by your state weed specialists helps ensure that a different group number has activity on your target weeds.”

Mode Vs. Site of Action

Herbicide mode of action is defined as the physiological or biochemical process within the plant that is impaired or inhibited by the herbicide (or how the herbicide works). Within these mode of action groups, are site of action groups. The site of action is defined as the physical location within the plant where the herbicide binds (or where the herbicide works).

Once a weed is resistant to a site of action, herbicides within that same site of action will no longer work. The herbicide classification chart groups herbicides by their site of action to assist you in selecting herbicides to:

  1. Maintain greater diversity in herbicide use
  2. Rotate among effective herbicides with different sites of action to delay the development of herbicide resistance.

Sprague says it’s important to understand mode versus site of action. In the amino acid synthesis inhibitors, for example, two sites of action exist. It includes all the ALS Inhibitors, which are Group 2 herbicides, and glyphosate, which is a Group 9 EPSP Synthase Inhibitor.

“For example, if I only had ALS-resistant common ragweed, and I looked only at the mode of action, I might conclude that glyphosate is under that mode, so it's not going to work. But since it’s a different site of action, it is still effective,” she adds.

Rotating herbicide site of action over the years can slow herbicide resistance. Weed scientists urge growers to avoid the same site of action in multiple applications made to the same crop in the same year.

“A good goal is to have two ‘effective’ sites of action on a particular weed," explains Sprague. "For example, if a premix contains two to three herbicides, but your weed is resistant to two of them, then you only have one ‘effective’ site of action. That puts extra selection pressure on the existing weeds—where they might develop resistance to that particular herbicide site of action group."

Herbicide Premix Challenges

Growers should also examine the actual rates of all herbicides within a premix. “In some premixes, you'll find a few lower rates than what we'd generally use. If you have a resistant waterhemp problem, for example, some of these reduced rates aren't high enough for adequate control,” Sprague says.

University weed control guides may include premix tables that outline individual herbicide rates. “This knowledge is critically important, especially if you're relying on that one effective site of action to control a particular weed,” she adds. Check the weed control guide’s efficacy or effectiveness tables to know which herbicides offer excellent, good, fair, poor or no control on specific weeds.

The herbicide classification guide is updated every year since Sprague and Aaron Hager, University of Illinois weed specialist, began 20 years ago to simplify herbicide groups when they worked together at the University of Illinois. “We appreciate continued funding of the Take Action program by the Soy Checkoff (United Soybean Board) and input from many university Extension weed scientists," she says.

For more details on using the Take Action chart, download this PDF.

Content provided by DTN/Progressive Farmer