Herbicide Mode of Action Groups 101

The terms “mode of action” and “site of action” are often used interchangeably when referring to herbicide classifications, but they’re not exactly the same. Understanding the difference between a herbicide’s mode of action and its site of action can help you build a successful weed control program while also managing herbicide resistance.

In this post, we will explore:

• What’s the Difference Between Site of Action and Mode of Action?
• Why Is Mode of Action Important?
• Mode of Action and Herbicide Resistance
• 9 Herbicide Modes of Action Groups
  • Lipid Synthesis Inhibitors
  • Amino Acid Synthesis Inhibitors
  • Growth Regulators
  • Photosynthesis Inhibitors
  • Nitrogen Metabolism Inhibitors
  • Pigment Inhibitors
  • Cell Membrane Disruptors
  • Seedling Root Growth Inhibitors
  • Seedling Shoot Growth Inhibitors

What’s the Difference Between Site of Action and Mode of Action?

Think of the site of action as the “where” and the mode of action as the “how” herbicides work. Herbicides are classified into site of action groups based on the particular enzyme or metabolic process they disrupt in a weed.

Multiple site-of-action herbicide groups may fall into broader mode-of-action groups, which describe how the herbicides disrupt weed growth and development. For example, Group 2 ALS inhibitors and Group 9 EPSP synthase inhibitors disrupt different weed enzymes, but they fall under the same mode of action group, amino acid synthesis inhibitors.

Why Is Mode of Action Important?

Understanding a herbicide’s mode of action can help you choose the most effective product for managing a weed based on its unique biology and life cycle. It can also help diagnose crop injuries following a herbicide application.

However, understanding the herbicide mode of action is most important for managing herbicide resistance and stewarding effective chemistries so they have a longer lifespan.

When your weed management program includes herbicides with multiple modes of action, you can:

• Mitigate risk
• Achieve complete weed control more consistently
• Delay weed resistance in your fields

Mode of Action and Herbicide Resistance

You may have heard that you should rotate herbicide modes of action to reduce the risk of developing herbicide-resistant weed populations. The science behind herbicide resistance can be complex, but here’s a simplified explanation.

Imagine you have a field that contains waterhemp. Basic biology tells us that some of the waterhemp weeds in your field have innate genetic tolerance to Herbicide A. When you spray Herbicide A, most of the waterhemp plants will die, but those with natural tolerance to the herbicide will not. In turn, those more tolerant weeds will reproduce and spread their genes, building up the population of resistant weeds. Herbicide A eventually becomes ineffective on a majority of the weed population because it’s been relied upon too heavily for control.

Now, consider the same field where you use multiple herbicide modes of action to manage weeds. You’ve added Herbicide B and Herbicide C to your tank mix. Those waterhemp weeds with natural resistance to Herbicide A’s mode of action are controlled using the alternative modes of action from Herbicides B and C. The result is more consistent weed control, reducing the ability of herbicide-tolerant weeds to reproduce.

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