Common Herbicide Mode of Action Groups for Weed Management

Tracy Pell

May 08, 2020

It’s easy to get overwhelmed when you start to plan for weed control. The list of products is long and the chemical formulations all start to sound the same after a while. So, when it’s time to go into the field, how do you select the herbicides that are best for your operation within each spraying window?

How Should Farmers Select Herbicides?

One of the biggest things to consider when choosing herbicides for your weed management program is each product’s mode of action. Mode of action explains specifically how the herbicide affects the plants normal growth and plant processes to kill the weed.

Understanding herbicide mode of action (MOA) groups can help you get the most out of your weed management program. Using multiple modes of action allows for the most comprehensive control, and just as importantly, using a variety of modes of action helps prevent weeds from becoming resistant to any particular mode of action.

Let’s take a non-GMO soybean crop for example.

Most Commonly Used Herbicide Modes of Action:

  • ACCase Inhibitors (Group 1): These products are used strictly for grass control, making them useful primarily in broadleaf crops. Examples: Fusilade® DX, Select Max®

  • ALS Inhibitors (Group 2): ALS inhibitors make up the largest mode of action. Many herbicides in this mode of action fall into one of two chemical families: imidazolinones or sulfonylureas. Examples: Classic®, Valor® XLT

  • Growth Regulators (Group 4): This mode of action, also known as synthetic auxins, includes many plant hormone-type herbicides. These are generally selective for broadleaf control in grass crops, but can be used for preplant and in-season weed control in broadleaf crops. Examples: 2,4-D Amine 4, Clarity®

  • Photosynthesis Inhibitors—Photosystem II (Groups 5 & 6): These inhibit photosynthesis and are used for control of grass and broadleaf weeds. Examples: Trivence®, Basagran®

  • Aromatic Amino Acid Inhibitors (Group 9): Glyphosate makes up this entire category. It has multiple formulations and can severely injure or kill any living plant tissue that it comes in contact with. Examples: Roundup PowerMAX®

  • Glutamine Synthesis Inhibitors (Group 10): This category covers glufosinate. Glufosinate can be used as a non-selective burndown treatment. Examples: Liberty®

  • PPO Inhibitors (Groups 14): PPO inhibitors are also known as cell membrane disruptors. Some can be applied pre-emergence, but most are used for post-emergence control. Examples: Sharpen® powered by Kixor®, Flexstar®

  • Shoot Growth Inhibitors (Groups 15): Herbicides in this group control weeds that have not emerged from the soil surface, generally grass and small-seeded broadleaf weeds. Examples: Dual II Magnum®, Zidua®

  • Photosynthesis Inhibitors—Photosystem I (Group 22): Photosystem I inhibitors are used for non-selective weed control and crop desiccation prior to harvest. Examples: Gramoxone®

The details matter when it comes to herbicide mode of action, so be sure to read product labels and consult with your county extension office if you’re unsure about any potential for crop injury from a spray.


Always read and follow label instructions. Some crop protection products may not be registered for sale or use in all states or counties.

Select Max® and Valor® are registered trademarks of Valent U.S.A. Corporation. Dual II Magnum®, Flexstar®, Fusilade® and Gramoxone®, are trademarks of a Syngenta Group Company. Liberty® is a registered trademark of Bayer. Trivence® and Classic® are trademarks or registered trademarks of DuPont or its affiliates. Basagran®, Clarity®, Sharpen® powered by Kixor® and Zidua® are registered trademarks of BASF. Roundup PowerMAX® is a registered trademarks of Monsanto Technology, LLC.

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Tracy Pell

May 08, 2020