Fight Herbicide Resistance: Mode of Action versus Site of Action

Varying mode and site of action is one way to manage herbicide resistance.

Herbicide-resistant weeds have become a significant challenge for U.S. crop production. One way to help manage and fight herbicide resistance is to vary herbicide modes of action (MOA) used on the same acre. We’re also hearing about the need to vary sites of action. What’s the difference between mode of action and site of action?

The simple answer is that both “mode” and “site” have a place in the resistance management conversation. In terms of plant response, site of action refers to the biological location on the target plant that is affected. The herbicide mode of action is what affects the biological site of the target plant.

There are 11 primary herbicide modes of action. These categorize more than 79 active ingredients that affect various sites of action in the weeds farmers want to control.

Herbicide labels include group numbers to help growers and applicators vary herbicide modes of action.

The Weed Science Society of America (WSSA) and DuPont Crop Protection promote voluntary MOA labeling and a MOA classification system (PDF).  WSSA has categorized herbicides by MOA and assigns each category with a group number, explaining how each herbicide’s active ingredient works on a specific biological site of action. Some DuPont herbicide labels include more than one group number to help growers and applicators vary herbicide MOA in tank mixtures and herbicide rotations. XX and YY are used as placeholders in the examples below.

To help prevent spread of resistant weed biotypes, keep field records that identify the weeds present and the herbicide names, MOAs and group numbers applied to each field.
