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United States Department of Agriculture

Agricultural Research Service

Title: Classes and Modes of Action of Herbicides

Author
item Boydston, Rick

Submitted to: Washington State Weed Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: September 1, 2000
Publication Date: November 1, 2000
Citation: BOYDSTON, R.A. CLASSES AND MODES OF ACTION OF HERBICIDES. PROCEEDINGS OF THE 50TH ANNUAL WASHINGTON STATE WEED CONFERENCE, P. 59-62. 2000.

Interpretive Summary: Herbicides can be classified in several different ways, such as, site of uptake in the plant, degree of translocation within the plant, time of application, chemical structure similarity, and mode of action. This presentation discusses herbicides commonly used in Washington State grouped according to mode of action. Mode of action is the sequence of events in which an herbicide kills plant. The site of action or target site is the particular plant function that is affected by the herbicide. Knowledge of an herbicide's mode of action can be useful in helping understand herbicide resistance and designing strategies to delay or prevent buildup of herbicide resistant weed populations. Herbicides with the same mode of action should be rotated or tank mixed with herbicides with different modes of action in crop rotations to prevent continuous selection of naturally occurring herbicide resistant weeds. Herbicides that inhibit different functions or enzymes in the plant often cause distinct injury symptoms. Knowledge of an herbicide's mode of action and injury symptoms can be useful in resolving problems with herbicide carryover or drift.

Technical Abstract: Herbicides with similar target sites and modes of action can be grouped to help growers better manage herbicide resistant weed populations by rotating herbicides with different target sites. Many herbicides commonly used in the Pacific Northwest can be grouped into the following categories: acetyl CoA carboxylase (ACCase) inhibitors, acetolactate synthase (ALS) inhibitors, microtubule assembly inhibitors, synthetic auxins (phenoxys, benzoics, picolinic acids), photosystem II inhibitors, glutamine synthetase inhibitors, protoporphyrinogen oxidase (Protox) inhibitors, and chloroacetamides. Herbicides within each group should be rotated with herbicides with different modes of action to delay or prevent buildup of herbicide resistant weed populations. In the Pacific Northwest, Prickly lettuce, Russian thistle, and kochia have developed resistance to herbicides that inhibit the ALS enzyme. Pigweed, common lambsquarters, common groundsel have developed resistance to herbicides that inhibit photosystem I. Wild oat populations have developed resistance to both ACCase and lipid synthesis inhibitors. Continuous use of herbicides with the same mode of action in the crop rotation has contributed to the buildup of these herbicide resistant weed populations.

Last Modified: 10/22/2014
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