There’s no shortage of herbicide choices for growers in the marketplace today. Old and new products, reformulated products, top brands, generics . . . the number of choices can be overwhelming if you’re trying to find a solution for a weed problem.

But understanding what options are available is important, especially if you’re goal is to develop and implement an effective weed management program. A good place to start is becoming familiar with the science of how herbicides work and, in turn, recognizing the difference between “mode of action” and “site of action.”

In order to be effective, herbicides must do the following:

  • have adequate contact with the targeted plant;
  • be absorbed into the plant;
  • move within the plant without being deactivated; and
  • reach an adequate level of toxicity.

In short, this process describes the herbicide’s mode of action, or the sequence of events that begins with the plant absorbing the chemical and ends with the targeted plant being injured or killed.

On the other hand, the herbicide’s site of action describes the specific biochemical site within a plant the herbicide directly affects, thus negatively impacting plant growth and development.

“Mode of action is a more overarching, comprehensive term, as it includes all the interactions between herbicides and plants,” explained Dallas Peterson, a professor and weed science specialist with Kansas State University.

“Site of action is the very specific biological process in the plant that the herbicide interferes with or disrupts, like a certain enzyme or protein,” he continued.

Knowing the difference

Though acknowledging mode of action and site of action are often interchanged, Peterson said the difference between the two is important to understand because relying heavily on herbicides with the same site of action increases the risk of developing herbicide resistance. Given this, reading and following herbicide application directions is a critical part of your weed management program, as is making sure you don’t overuse a particular site of action.

“We recommend using different sites of action, as opposed to relying on just one,” said Peterson. “Simply rotating among herbicides with the same site of action won’t help with resistance management.”

According to Peterson, one of the best ways for growers to not get too reliant on a particular site of action is to practice crop rotation, which allows for the introduction of other chemicals. Utilizing cultural practices and paying close attention to the proper (and timely) use of herbicides are two other keys to success when it comes to weed management.

 “We’re creatures of habit, so when we find something that works we use it, especially if it’s cost effective. Roundup Ready technology is an example of this because it has been an economical and reliable program,” said Peterson, while explaining how overreliance on one site of action option can be problematic.

Bottom line, becoming educated and understanding how sites of action are classified according to the injury symptoms they create in particular plants is not only a good first step in creating an effective weed management plan, but also critical to reducing the risk of developing herbicide-resistant weeds over the long-run.

“Incorporating different sites of action is important, because anything we can do to diversify our weed management practices is good,” said Peterson.

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