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Keeping Weeds DOA: A New SOP for Herbicide MOA/SOA

Aaron Hager Aaron Hager
Extension Specialist
Department of Crop Sciences
(217) 333-4424; hager@illinois.edu
Christie Sprague
  Christie Sprague
Assistant Professor
Department of Crop Sciences
(217) 333-4424; csprague@illinois.edu

Individual herbicides may be grouped or classified into families according to common characteristics. Examples of herbicide families include classification according to application timing (PPI, PRE, and POST), chemical structure (imidazolinones, chloroacetamindes, etc.), and mode of action (photosynthetic inhibitor, growth regulator, etc.). While exceptions to any classification scheme likely will always occur, classifying herbicides into families can provide useful frameworks for understanding general commonalities.

Cases of herbicide-resistant weed biotypes worldwide.
Figure 1. Cases of herbicide-resistant weed biotypes worldwide.

Herbicide-resistant weeds (by 1983) according to herbicide mode of action.
Figure 2. Herbicide-resistant weeds (by 1983) according to herbicide mode of action.

Herbicide-resistant weeds (1984 to 1999). Note the increase since 1983 in the number of herbicide-resistant weed biotypes.
Figure 3. Herbicide-resistant weeds (1984 to 1999). Note the increase since 1983 in the number of herbicide-resistant weed biotypes.

An appreciation of how and where herbicides work in plants can be valuable for planning weed management strategies that reduce the potential to select for herbicide-resistant weed biotypes. Herbicide mode of action (MOA) may be defined several ways, but it essentially describes how the herbicide works to inhibit plant growth. Herbicide site of action (SOA), however, is the physical location within the plant where the herbicide binds.

Several herbicide MOA classification schemes have been proposed, but unfortunately there can be a lack of uniformity among these schemes with respect to how the families are named and which herbicides are grouped into which families. MOA family names such as "growing point disintegrator" and "cell membrane disrupter" have been used, but these names do not provide much detailed information about how or where the herbicides work within the plant.

While an understanding of herbicide MOA is beneficial, classification of herbicides by their site of action may prove to be a more reliable classification scheme to delay the development of herbicide-resistant weed biotypes. For example, classifying herbicide MOA as "inhibition of amino acid biosynthesis" places Roundup and Pursuit in the same family, whereas classification by SOA places these two herbicides into distinctly different families.

The development of herbicide-resistant weed biotypes is the result of sufficient selection pressure imposed on the weed population by repeated use of herbicides with a similar mode/site of action. The frequency of herbicide-resistant weed biotypes continues to increase in Illinois, with nine confirmed herbicide-resistant biotypes to date. Worldwide, the problem also has dramatically increased during the past decade (Figure 1). Resistant weed biotypes have developed to several herbicide chemical families, and the proportion of resistant weeds to various herbicide chemical families has fluctuated over time (Figure 2 and Figure 3). While arguments persist about which strategies are better able to reduce the selection pressure for herbicide-resistant weed biotypes, understanding where herbicides act within the plant remains the first step to successful management of this problem.

The Weed Science Society of America (WSSA) and the Herbicide Resistance Action Committee (HRAC) have developed classification schemes based on herbicide site of action. While minor differences exist between the two schemes, they both convey essentially the same information.

We have adapted the WSSA herbicide site of action classification scheme into a color-coded system where herbicides with similar target sites are outlined in similar colors. Herbicide premixes also are classified by target site(s), with identical or different colors where appropriate. This chart is adapted from Retzinger and Mallory-Smith (Weed Technology 11:384-391).

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