April 2002 // Volume 40 // Number 2 // Tools of the Trade // 2TOT4

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A Contemporary, Color-Enhanced Herbicide Site of Action Bulletin

Incidences of herbicide-resistant weed biotypes continue to increase throughout the Midwest. Management approaches to reduce the selection of herbicide-resistant weed biotypes include rotating herbicides based on modes or sites of action. The University of Illinois Extension bulletin, "Utilizing Herbicide Site of Action to Combat Weed Resistance to Herbicides," establishes a classification based on 14 sites of action with each individual site of action coded with a distinct "primary" color. This bulletin is intended to enhance the ability of growers to rotate herbicides based on site of action to slow further development of herbicide-resistant weed biotypes.

Aaron G. Hager
Weed Science Extension Specialist
Internet Address: hager@uiuc.edu

Christy L. Sprague
Assistant Professor
Internet Address: csprague@uiuc.edu

University of Illinois
Urbana, Illinois


Herbicides are an integral part of many weed management systems in Midwestern states. Repeated use of herbicides that act in a similar manner within the target weed has resulted in the selection of weed biotypes that are resistant to these herbicides. The Weed Science Society of America (WSSA) defines herbicide resistance as the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type. The number of herbicide-resistant weed biotypes continues to increase in the United States and worldwide. Heap (2001) reports 252 herbicide-resistant weed biotypes occur in 47 countries worldwide.

Herbicide Classification by Site of Action

The WSSA has developed a herbicide classification based on herbicide site of action. Terminology used to describe herbicide site of action is often cumbersome for growers to comprehend, so we have adapted it into a color-enhanced system where herbicides with similar target sites have similar colors. Herbicide premixes are also classified by target site(s), with identical or different colors where appropriate. This color-coded system is adapted from the site of action classification outlined by Retzinger and Mallory-Smith (1997).

The University of Illinois Extension bulletin, "Utilizing Herbicide Site of Action to Combat Weed Resistance to Herbicides," establishes a color-coded herbicide site of action classification system based on 14 sites of action. This three-page bulletin is intended to enhance the ability of growers to rotate herbicides based on site of action to slow further selection for herbicide-resistant weed biotypes.

The front cover explains the importance of using a site of action classification for herbicide resistance management. The inner table (Figure 1) separates herbicide sites of action into 14 "primary" colors. Herbicide chemical families sharing a particular site of action are coded in shades of the respective site of action family "primary" color.

Figure 1.
A Color-Coded Table of "Utilizing Herbicide Site of Action to Combat Weed Resistance to Herbicides"

An Example of a Color-Coded Herbicide Classification Table

The bulletin also includes common and trade names of many herbicides used in Midwest agronomic production systems. The back page of this bulletin includes corn and soybean herbicide premixes, with individual premix components coded with the appropriate color based on their respective site of action.


The development of herbicide resistance in weed populations can result in significant economic losses for growers. Growers, however, frequently continue to use a successful herbicide program until it fails instead of proactively implementing herbicide resistance management strategies.

Peterson (1999) suggested the greatest economic loss producers face due to selection of herbicide-resistant biotypes likely occurs during the first year of poor weed control. Shaner (1995) suggested that the long-term economic consequences of herbicide resistance include loss of herbicide performance and shifts in weed populations. Orson (1999) argued that preventing the selection of herbicide-resistant weed biotypes can often cost a producer significantly less than the costs incurred dealing with resistance once it has developed.

Extension and private industry have proposed numerous management strategies to retard the selection for herbicide-resistant weed biotypes, including utilizing nonchemical weed management options (such as mechanical cultivation), crop scouting and rotation, herbicide tank-mixtures, and rotation of herbicides that act in dissimilar fashions (Shaner 1995). Labels of herbicides registered for use in the United States generally do not enumerate the herbicide site of action. If growers elect to implement herbicide rotation or tank-mixtures as a resistance management strategy, information is needed to identify which herbicides act in a similar manner.

Herbicides are frequently categorized into families according to various similarities. Examples of herbicide classification categories include mode of action, application timing, and chemical structure. Herbicide mode of action describes the metabolic or physiological plant process impaired or inhibited by the herbicide. Essentially, mode of action refers to how the herbicide acts to inhibit plant growth. Herbicide site of action describes the specific location(s) within the plant where the herbicide binds. Site of action thus identifies the herbicide target site within the plant. The most common herbicide classification schemes utilize mode of action; however, much ambiguity exists with respect to herbicide classification based on mode of action.

While an understanding of herbicide mode of action is beneficial, classifying herbicides by site of action may be a more useful system from a resistance management standpoint. Herbicide resistance in plants is often due to an alteration of the binding site in the target plant. Rotating herbicides based on these different binding site(s) or site(s) of action may provide a more reliable classification system. As previously mentioned, classifications based on herbicide mode of action are rather ambiguous. For example, classification systems based on mode of action include anywhere from seven to 13 different categories. Some of these systems describe mode of action categories as "cell membrane disruptors," "seedling growth inhibitors," and "amino acid synthesis inhibitors." Rotating herbicides based on these categories could cause confusion among growers. For example, the mode of action category "amino acid synthesis inhibitors" would place the herbicides Pursuit (imazethapyr) and Roundup (glyphosate) in the same family, whereas classification by site of action would place these two herbicides into two distinctly different families, allowing growers to more accurately rotate herbicides for resistance management.


Each year the frequency of herbicide-resistant weed biotypes continues to increase in the Midwest. This, coupled with the decreased development of herbicides with new active ingredients, has made it extremely important to manage current herbicides through integrated management practices. Without the proper strategies in place to delay the selection of herbicide-resistant weeds, growers in the Midwest will likely be faced with fewer weed management options and long-term economic consequences. This bulletin could be considered in an overall herbicide resistance integrated management approach to prevent proliferation of herbicide resistant weeds.

To obtain a copy of the bulletin "Utilizing Herbicide Site of Action to Combat Weed Resistance to Herbicides," contact: Information and Technology Communication Services, University of Illinois, Marketing and Distribution, 1917 South Wright Street, 61820, 1-800-345-6087, acespubs@uiuc.edu.


Heap, I. (2001). International survey of herbicide resistant weeds [On-line]. Available at: http://www.weedscience.org/in.asp

Orson, J. H. (1999). The cost to the farmer of herbicide resistance. Weed Technol. 13:607-611.

Peterson, D. E. (1999). The impact of herbicide-resistant weeds on Kansas agriculture. Weed Technol. 13:632-635.

Retzinger Jr., E. J. & Mallory-Smith, C. 1997. Classification of herbicides by site of action for weed resistance management strategies. Weed Technol. 11:384-389.

Shaner, D. L. (1995). Herbicide resistance: Where are we? How did we get here? Where are we going? Weed Technol. 9:850-856.