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Herbicide mode of action and sugarbeet injury symptoms

Sugarbeet is one of several crops within Beta vulgaris. Sugarbeet has evolved through time from a labor-intensive agricultural crop with static yield to one that is highly mechanized and with steadily improving yield.

Weeds have been a significant production challenge in sugarbeet since the crop first was cultivated in Europe in the late 1700s. Interference from uncontrolled weeds can suppress sugarbeet so severely that no crop is produced. Weeds that emerge within the first eight weeks after planting especially influence sugarbeet yield.

Herbicide application timing

Herbicides are applied alone or in mixtures before planting (preplant), immediately after planting (pre-emergence), after sugarbeet has emerged but before weeds have emerged (lay-by), and after sugarbeet and weed emergence (postemergence). Injury can occur from herbicides applied to sugarbeet for weed control, and from off-target movement of herbicides applied to other crops in adjacent fields or herbicide applied in previous years’ crops that carryover to sugarbeet.

How herbicides work

Herbicide efficacy depends on the morphology and anatomy of the plant and many physiological and biochemical processes that occur within the plant, including:

  • Droplet retention.
  • Herbicide deposition.
  • Translocation (movement) of active herbicide within the plant.
  • Toxic levels reaching the site of action, such as a specific enzyme, or the plant processes that are disrupted by the herbicide

The application method, whether preplant incorporated, pre-emergence or postemergence, largely determines when the herbicide will contact plants and the portion of the plant contacted.


Herbicide families

An understanding of the way herbicides act to kill weeds (herbicide mode of action) is useful in selecting and applying the proper herbicide for a given weed control problem. Herbicide mode of action information also is useful in diagnosing injury from herbicides.

Although many herbicides are available, they can be categorized into groups with similar chemical and phytotoxic (plant injury) properties. The Weed Science Society of America (WSSA) has developed a numbered classification system based on the herbicide site of action or the specific plant process disrupted by the herbicide.

Knowledge of herbicide sites of action allows proper selection and rotation of herbicides to reduce the risk of developing herbicide-resistant weeds.

The following webpages describe the characteristics of widely used herbicide families grouped by mode of action and the WSSA classification number (in parentheses). These eight major modes of action are:

Nonherbicide injury symptoms


Terms and Herbicide Classification


Revised by

Thomas J. Peters, Extension sugarbeet specialist, University of Minnesota and North Dakota State University; Michael S. Metzger, Research agronomist, Minn-Dak Farmers Cooperative, Wahpeton, N.D. and Peter J. Regitnig, Research agronomist, Lantic Inc., Taber, Alberta, Canada

Authored by

Alan Dexter, former Extension sugarbeet specialist, North Dakota State University; Jeffrey Gunsolus, Extension weed scientist and William Curran, Extension agronomist-weed control emeritus, Pennsylvania State University. 1994

Photos provided by Thomas Peters, University of Minnesota and North Dakota State University unless otherwise noted.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.

Reviewed in 2019

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