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Know your site of action

Know your site of action

Mixing and matching weed control tech is easier if you know the rules.

In today’s farming environment, where multiple factors can impact your bottom line, the term “site of action” could potentially come up during a discussion over weed control and herbicide use.

But even if site of action isn’t a hot topic of conversation between you and your farming friends, a basic understanding
of the science behind it is something all growers should have in their farm management toolbox. The first step is to understand there’s a difference between a herbicide’s mode of action and site of action, as they’re not interchangeable terms.

Defining terms

Every weed has a weakness or two, and different herbicides target different parts of the plant for control. Knowing the approach a herbicide takes can help you avoid weed resistance.

Mode of action is the sequence of events where the herbicide targets a particular biochemical process in plants (like photosynthesis), which then causes injury or death. Site of action describes the specific biochemical sites within a plant that the herbicide directly affects, which then negatively impacts the plant’s growth and development. Simply put, it’s a micro vs. macro comparison, as site of action is a subgroup within mode of action.

“We used to use the term ‘mode of action’ a lot back in the 1980s and 1990s. However, we’re now dealing with herbicide-resistant weeds, so we’ve had to drill down to sites of action because mode of action is a more general process,” says William Curran, a professor of weed science at Penn State University.

Curran says there are eight to 10 different modes of action and about 30 different sites of action; and each site of action has been assigned a numerical value, as part of an industrywide standard. Given this, every herbicide label lists a site of action number that signifies to users how that particular herbicide, if used properly, negatively impacts unwanted plants. However, despite the merits of this identification system and the availability of herbicides addressing many different sites of action, Curran said it’s far from a perfect process.

“No site of action is fail-safe, or 100% successful,” notes Curran, who says a misconception among growers is that some herbicide families are immune to developing resistance.“The analogy would be what occurs with pharmaceuticals and antibiotic-resistant germs in humans, as there are germs that have developed resistance to certain drugs, just like some weeds have developed resistance to certain herbicides.”

Given this, Curran advises growers not only to learn about sites of action and understand how the science fits into weed control, but also to be more strategic when it comes to the sites of action they’re using both short and long term.

“If you’re using something that has multiple sites of action, it makes it harder for weeds to develop resistance, but it can still happen,” just like with 2,4-D-resistant waterhemp, he says.

“Rotating sites of action and not using the same site of action from year to year is a good step in helping prevent resistance over the long term.”

Given the ongoing threat of herbicide-resistant weeds, Curran believes this issue has captured the attention of growers. “I think farmers are really starting to understand sites of action and are using it to their advantage in their own weed control efforts.”

For more information about site of action and resistant weeds, check out the United Soybean Board’s new website —

Yontz writes from Urbandale, Iowa.

Solution Center is independently produced by Penton Farm Progress through support from SureStart® II herbicide. For more information, visit

View more Solution Center content on Farm Progress.

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