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  • Writer's pictureRachel Stevens

Modes of Action--The important things you need to know.

Updated: Feb 15, 2021

We spend a lot of time picking out chemistries, selecting the best combinations of pre and post herbicides, determining timing and rates, and rightfully so. Managing weeds is an essential part of our agronomic system and can be incredible yield limiters if we don’t keep control of them. We’ve seen that as we have more weeds with resistances and are forced to rely more extensively on more costly chemicals. I want to spend a little time today talking about modes of action and the action groups they each belong in.

First of all, modes of action refer to the functional or anatomical change by a living organism, when it is exposed to a substance. In this case, it would be how the plant or weed changes, or what the impact is on plant growth in response to a herbicide. Herbicides with the same mode of action will result in the same movement of herbicides through the plant system as well as similar looking injury.

Secondly, sites of action refer to which process in the plant the different modes of action are disrupting. Each of these are named to indicate what specific implication it has, like ALS inhibitors or photosystem II inhibitors of HPPD inhibitors. Each site of action can be grouped into a mode of action group like photosynthesis inhibitors, or amino acid synthesis inhibitors.

Here are a few more things you need to know.

There are 10 commonly used modes of action:

1. Lipid synthesis inhibitors

2. Amino acid synthesis inhibitors

3. Growth regulators

4. Photosynthesis inhibitors

5. Nitrogen Metabolism inhibitor

6. Pigment inhibitor

7. Cell membrane disrupters

8. Seedling root growth inhibitors

9. Seedling shoot growth inhibitors

10. Undefined

Here is an excellent chart outlining the modes of action and sites of action.

In order to prevent resistance from developing, we need not only diversify outside the site of action group, but also outside our modes of action group. By mixing up modes of action, a plant that had grown resistant to a certain method of attack will likely be susceptible to another mode of action. This is a critical part of weed control, and selecting a variety of products throughout the year and from year to year to mix up modes of action is critical. When is the last time you thought through the different changes in chemistry that you’ve been using? Have you used several from the same action site? It’s time to start tracking modes of action and prevent yourself some trouble down the road.

You may wonder why I place such an emphasis on changing modes of action. The reason is this: there have been no new modes of action discovered since the mid 1980’s. YES. 35 years ago. Because of this, we need to work hard to preserve the modes of action we do have available. Some of these sites of action already have as many as 45 resistant weeds. Of the 26 known sites of action, 23 have weed resistances verified. We need to put in the work now to make sure we are rotating chemistries and modes of action. We also need to be following robust application guidelines and getting the correct rate and GPA on every time. But that’s a whole different blog post.

A look at some of our soybean pre plant chemistries

A lot of these products, regardless if you are selecting a product from BASF, FMC, Corteva, or Helena are a combination of multiple products that each have different modes of action. For example, the product Kyber from Corteva would have the same ingredients as Valor, Metribuzin, and Zidua SC, which have the group numbers 14, 5, and 15. In this case, 14, 5, and 15 are within three different modes of action, cell membrane disrupters, photosynthesis inhibitors, and seedling shoot growth inhibitors. This variety of modes of action is essential to preventing resistances. A look at some of our other recommended pre plant chemistries also show multiple modes of action with groups 14, 5, and 15, and a few also using group 2 chemicals.

If you have any questions about modes of action, or resistances you might be seeing on your farm, or questions about pre plant soybean chemistry, drop a question in the comments below, or shoot us an email.

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