The Enid News and Eagle, Enid, OK

December 1, 2012

You need to know herbicide’s mode of action

By Roger Don Gribble
Enid News and Eagle

ENID, Okla. — There are a large number of herbicide options available to crop producers, gardeners and landscapers.

We have new products, old products with new names, and new formulations of old products, premixes and now generics. Can protecting your desired plants from weeds get any more complicated? Yes it can.

You will need to know the plants a product can be used with, and now, will the product you choose damage the weed causing you problems without hurting the desired plants that are growing? It will be more important to know how the product of choice will harm the weed of choice. This means you need to know the “mode of action” of the herbicide.

The mode of action of an herbicide is the way in which the herbicide controls the susceptible plants within a field or landscape. It usually describes the biological process or enzyme in the plant the herbicide interrupts, affecting normal growth and development. The mode of action also might be a general description of the injury symptoms seen on a susceptible plant.

In Oklahoma, there are 11 modes of action commonly used. There are more, but the others seldom are used in our state. Each is unique in the way it works on susceptible plants. Some modes of action comprise several chemical families that vary slightly in their chemical composition but cause similar injuries to susceptible plants.

Knowing and understanding each herbicide’s mode of action is an important step in selecting the proper herbicide and designing a successful weed management program. Right now, our over-reliance on a single herbicide mode of action is placing heavy selection for resistant weeds. Over time, resistant weeds multiply and become the dominate weed in the field or landscape, resulting in herbicides that are no longer effective. Rotating herbicide modes of action, along with other weed control practices, will prevent or delay the development of these resistant individuals.

Information regarding each herbicides mode of action can be found on the herbicide label. Often times, you will see an herbicide being a member of a particular numbered group. These numbers refer to a specific mode of action and were developed to consistently organize herbicides based on their mode of action.

For example, “Group 1” herbicides are ACCase inhibitors. “Group 2” is ALS inhibiting herbicides and so on.

In some situations, herbicides may not be listing their mode of action. If this is the case, contact your county extension educator for clarification. He or she will direct you to Extension Fact Sheet PPS-2778, “Understand-ing Herbicide Mode of Action.”

This fact sheet will list the group number, chemical family and trade names for almost all of our commonly used herbicides. Most importantly, the fact sheet lists the active ingredient of the herbicide. With the emergence of the generic formulations of herbicides, trade names we have become accustomed to seeing have changed, but their active ingredient name stay consistent. Please get your copy from your Oklahoma Cooperative Extension Service educator soon.

Gribble is Oklahoma Cooperative Extension Service northwest area agronomist.