Herbicide:Mode of Action

Bob Scott: I’m Bob Scott, Extension Weed Scientist with the University of Arkansas, Cooperative Extension Service, Division of Agriculture. Today I would like to discuss with you herbicide mode-of-actions. Herbicide mode-of-action simply refers to the way in which a herbicide controls weeds. Knowing your herbicide modes-of-action has become more important over the past few years due to development of herbicide-resistant weeds. Specifically, resistant Palmer pigweed or Palmer amaranth in Roundup Ready soybeans have driven the need to use residual herbicides in Roundup Ready soybeans, conventional soybeans or Liberty Link soybeans.
Certain herbicides can have a different trade name and even a different common name but still be the same herbicide mode-of-action. Therefore, understanding herbicide mode-of-action is critical to the development of a resistance-management plan for resistant weeds in soybeans.
For your convenience, in the MP44, herbicide mode-of-action is now listed in the first column of the Weeds Control table for each herbicide section. For example, on page 36 of the Weed Response Ratings for Soybean Herbicides the first column is Herbicide Family. Herbicide Family is represented by a simple number. This number, again, refers to the way in which that herbicide controls weeds. The important thing is when developing a resistance-management plan is to use as many herbicides with different numbers that are still effective on the weeds that you have in your field. This publication is available at your county office or on the web at
Your Arkansas Soybean Podcast is a production of the University of Arkansas System, Division of Agriculture, and was funded in part by the Arkansas Soybean Promotion Board. For more information on soybean farming in Arkansas contact your local county extension office.