Dakota Farmer

Herbicide know-how: What's the difference between mode of action and site of action?

Herbicide mode of action and site of action are both important if you hope to prevent the spread of herbicide-resistant weeds.

September 16, 2016

2 Min Read

To prevent weeds from developing populations resistant to the herbicides you are using, you have to rotate herbicides. It’s important to know a herbicide’s mode of action (MOA) and site of action (SOA).Gared Shaffer, South Dakota State University Extension weeds field specialist, explains the difference.

MOA. All herbicide interactions with the plant, from application to final effect, are considered the MOA. The MOA involves absorption into the plant, translocation or movement in the plant, metabolism of the herbicide and the physiological plant response.


SOA. SOA is the specific process in plants that the herbicide disrupts to interfere with plant growth and development. The SOA is the most important aspect of herbicides when dealing with prevention and control of herbicide-resistant weeds.

The National Weed Science Society has numbered each group of herbicides under the same mode of action and site of action for ease of reference when planning your herbicide program.

Resistance management
There are 10 MOAs, but only eight of those are widely used, which in turn limits the available sites of action. There are over 700 registered herbicide products in South Dakota. Those products use eight modes of action with 18 sites of action.

Weed species that are resistant to three different SOAs — SOA 1, ACCase; SOA 2, ALS inhibitors; and SOA 9, EPSP Synthase inhibitor (International Survey of Herbicide-Resistant Weeds) — are present in South Dakota. More cases of resistance could be present but not yet documented.

To slow weed resistance, make sure to rotate herbicide sites of action within different modes of action as much as possible.

If you have suspected resistance on your land, contact SDSU Extension so the proper measures can be taken.

Source: SDSU Extension

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