Herbicide Control

Diversifying herbicide modes of action

Relying heavily on the same herbicide mode of action (MOA) from year to year causes selection of herbicide resistant weed populations in the field. This is because the resistant weeds survive repeated applications of the same herbicide every year, while susceptible weeds die off.

By diversifying the number of effective herbicide MOA in a tank and from one application to the next, the producer is limiting the opportunities for weeds to develop resistance.

Modes of action can and should be diversified several ways: In tank mixes, between applications within a season, and rotated between seasons. Rotating crops helps facilitate the rotation of herbicide MOA.

Additionally, it is important to use the full rate of each herbicide according to the label, as incomplete control can also lead to the development of resistance.

Video: Jason Norsworthy (University of Arkansas) explains the importance of using multiple herbicide modes of action (2:59).


In order to diversify MOA, the first step is to be aware of which are being used and which are effective on the weeds in the field. Since herbicides are often known by their trade names or common names, it is often hard to know which MOA it is. There are many trustworthy resources available for this information:

  • This Herbicide Classification Chart, published by Take Action, lists each herbicide by its trade name, common name (active ingredient), group number, and mode of action. It also tells how many weed species are resistant to it in the US.
  • Many university extension services also maintain published weed management guides that show the efficacy of numerous herbicides on many different weeds and crops, for both PRE and POST applications.
  • On this website, you can also hover over the “Search by State” tab to find your state and see a list of herbicide resistant weeds present and what groups they are resistant to.


How to Interpret Herbicide Mode of Action

Increased Dicamba Use Requires Enhanced Stewardship – Iowa State University

Understanding Herbicide Mode of Action – Oklahoma State University

Herbicide Resistance: Rotate Herbicide Modes of Action – Syngenta

Rotate Those Herbicide Modes of Action – Southwest Farm Press

Rotate Crops and Herbicide Modes of Action – WeedSmart.org

Weed Control Guide for Ohio, Indiana, and Illinois – by Mark Loux (OSU), Bill Johnson (Purdue), and Aaron Hager (U of IL)

Early Season Weed Control – Farm Progress

Strategies to Reduce Spray Drift – Kansas State University