The Value of Mode of Action

Understanding mode of action can help you manage herbicide resistance 

Farmers hear a lot about herbicide “mode of action” these days. But why is it important?  

Mode of action (MOA) is a simple but valuable tool developed by the Weed Science Society of America to help you plan a more diverse, sustainable and effective herbicide program. Here are some frequently asked questions about MOA: 

What is mode of action? 

While the terms mode of action and site of action are often used interchangeably, they are two different things. Herbicides are classified by their “site of action,” the specific biochemical site within the plant an herbicide binds to in order to disrupt the plant’s growth. The manner in which plant growth is disrupted, such as the type of injury inflicted on the cellular level, is known as the “mode of action.” 

How can MOA help me manage weed resistance? 

Repeated use of herbicides with the same MOA can result in herbicide-resistant weeds. To combat resistance evolution, growers should use MOA group numbers to plan a diverse, multi-year herbicide program that:  

  • deploysmultipleMOA against problem weeds, simultaneously and in sequence, and 
  • rotates among effective herbicides with differentMOAs, both annually and within a season. 

What are MOA group numbers? 

These numbers organize herbicides according to their MOA. All herbicides in a particular group act in a similar way to disrupt plant growth, even though they may belong to different chemical families. Within each MOA group, there are usually several chemical families, active ingredients and branded products. Resistant weed populations are defined by the MOA group number they are resistant to. 

Where can I find MOA group numbers? 

Group numbers are displayed on the first page of each herbicide label. You can also look up the MOA at TakeActionOnWeeds 

MOA group numbers will help you:  

  • includeas many modes of effective action as possibleagainst your worst weeds 
  • avoidoverusing any singleMOA 
  • understandwhichbranded herbicides have the same MOA and thus will not provide the necessary MOA diversification. 

Photo Credit: © iStock alffoto 2015

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