Get A Handle On Herbicides

Herbicides are critical to many successful agricultural operations. They work in different ways, and are often classified by their use, chemical family, mode of action, or type of vegetation controlled. An understanding of how herbicides work can help green industry professionals to select appropriate herbicides, and also to diagnose herbicide injuries. The use of suitable herbicides is critical in a weed management plan, in order to ensure satisfactory weed control while reducing likelihood of herbicide resistance. 

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Mode of Action

Herbicides kill plants by slowing or stopping critical enzymes or enzyme pathways that are necessary for plant life. Mode of Action refers to the injury that a herbicide causes to a plant. Herbicide injury is caused by interrupting or stopping some important plant process. Understanding a product’s Mode of Action allows us to understand how it will affect weeds as well as desirable plants in the case of accidental application or misuse.

The National Pesticide Information Center publishes technical fact sheets on pesticide active ingredients (http://npic.orst.edu/ingred/aifact.html) in which Mode of Action is discussed. Herbicides are usually grouped into eight Mode of Action families, which are discussed below. Mode of Action families group herbicides that cause similar injuries. The article will cover the following eight Modes of Action: 1) Amino Acid Synthesis Inhibitors; 2) Cell Membrane Disrupters; 3) Growth Regulators; 4) Lipid Synthesis Inhibitors; 5) Nitrogen Metabolism Inhibitors; 6) Photosynthetic Inhibitors; 7) Pigment Inhibitors; and 8) Seedling Growth Inhibitors.       

Site of Action

Categorized within each Mode of Action is one or more Sites of Action. An herbicide’s Site of Action describes the actual biochemical site (within a plant cell) where it works to cause injury. Sites of Action are described using very technical terms that indicate specific sites and pathways within plant cells that are affected by herbicides. 

The Weed Science Society of America (WSSA) assigns a group classification number from 1-29 to each site of action. [At the international level, the Herbicide Resistance Action Committee, or HRAC, classifies herbicides by lettered site of action. HRAC provides a reference chart with WSSA numbers and corresponding HRAC lettered classifications.] The EPA recommends that the WSSA number be displayed on herbicides, but this is voluntary, not mandatory. The WSSA group number is intended to serve as a tool to aid in herbicide selection.

Two examples of herbicide labels that display the WSSA classification include SureGuard Herbicide and Fusilade DX Herbicide. Look at these labels to become familiar with labeling of Site of Action. You will see Group (#) Herbicide at the top of labels that contain this information. 

Get A Handle On Herbicides

Site of Action is also called Mechanism of Action. This term is frequently, incorrectly, interchanged with Mode of Action. See the graphic above for a pictorial example of the relationship between these terms.  

Herbicide Families

Herbicide families are used to group herbicides with similar chemistries.  Within each of the families there are specific herbicidal active ingredients.    

  • Amino Acid Synthesis Inhibitors

Amino acids are the building blocks that make up a plants’ structure, and are formed by plants through oxygen and carbon from the atmosphere, and hydrogen and nitrogen from the soil. When amino acids combine, they form proteins. Amino acids are critical for plant health, growth, and water regulation.

Amino acid synthesis inhibitors act in some way to stop the critical process of amino acid formulation. Sometimes this Mode of Action is divided into two types: selective pre-emergent and non-selective, foliar applied. This family of herbicides includes WSSA groups #2 and #9 and acts slowly – injury symptoms will not show for one to two weeks. Injury symptoms include general stunting, chlorosis, purple veination of leaves, leaf crinkling, chloritic banding. One of the more common herbicides in this family is glyphosate (9), a non-selective amino acid synthesis inhibitor.    

Understanding Herbicides’ Mode Of Action To Maximize Your Weed Management Plan

  • Cell Membrane Disrupters

A plant’s cell wall provides structural support to cells. Chloroplasts, within plant cells, are responsible for conducting photosynthesis. The cell membrane regulates a cell’s fluid exchange, and allows a cell to hold cytoplasm, the fluid portion of a cell. Cell membrane disruptors are contact herbicides and damage plant cells by causing a plant’s cell wall and chloroplasts to collapse. This damage causes rapid wilting, water-soaked appearance, yellowing, and necrotic spotting. Drift injury may appear as necrotic speckling. On sunny days, injury can occur within just a few hours.   Roots are not affected by herbicides in this Mode of Action because products do not move through the plant. This family of herbicides includes WSSA groups  #14 and #22. One of the more common herbicides in this family is paraquat (22). 

  • Growth Regulators

The growth regulator family is a fairly large group that causes plant growth abnormalities in a number of ways through interference with biological activities such as respiration, cell division, and protein synthesis. These biological activities are disrupted through upsetting of normal hormone balances; hence the nickname of “hormone herbicides”. Injury symptoms may include leaf cupping, strapping, and crinkling, stem bending and brittleness. Growth regulators are usually systemic and often selective against broadleaf weeds. This family of herbicides includes WSSA groups #4 and #19. Common herbicides in this family include 2,4-D  (4), dicamba (4), and triclopyr (4). 

  • Lipid Synthesis Inhibitors

Plant lipids are important to cell membrane integrity and normal plant growth. Plants’ fatty acid creation is an important process in the development of plant lipids, which are fatty acids essential to plant function and structure and provide much of the structure essential to plant life. Lipid synthesis inhibitor herbicides prevent the creation of these important components. 

These herbicides are generally systemic and selective against annual and perennial grassy weeds. Lipid synthesis inhibitors work best on a healthy, vigorous grass, and will be less effective on already stressed weeds. The herbicide affects new growth first, causing yellowing of leaf tips, and slowed or stunted root and shoot growth. Leaf bases may develop a rotten appearance and may be easily removed from the plant. This family of herbicides includes WSSA group #1. One of the more common herbicides in this family is fluazifop (1). 

  • Nitrogen Metabolism Inhibitors

Plants metabolize nitrogen in the form of ammonia into other compounds beneficial to their growth, such as amino acids. Nitrogen metabolism inhibitor herbicides reduce or stop this process, causing a plant to suffer from ammonia toxicity and the deficiency of critical amino acids. This family of herbicides includes WSSA group #10. 

  • Photosynthetic Inhibitors

Photosynthesis is the process through which plants collect energy from the sun and convert it into the food used for plant growth. Photosynthetic herbicides shut down this process that is so important to plant survival.  These herbicides bind with specific sites within plants’ chloroplasts. This family of herbicides includes WSSA groups #5, #6, and #7. These contact herbicides are generally selective towards broadleaf weeds. Injury occurs quickly, first appearing as wilting, leaf tip yellowing, and blister like lesions that may become necrotic within twenty-four hours. One of the more common herbicides in this family is atrazine (5). 

  • Pigment Inhibitors

Chlorophyll is the green pigment in leaf tissue that is necessary for photosynthesis. Pigment inhibitor herbicides destroy chlorophyll and effectively stop photosynthesis. This family of herbicides includes WSSA groups #12 and #13. These herbicides are systemic and cause injury in the form of lightening or bleaching of whole leaves, or areas between veins.  Because of this, these herbicides are sometimes called “bleaching herbicides”. 

  • Seedling Growth Inhibitors

Seedling growth inhibitors are applied to the soil and inhibit either root development (WSSA group #15), shoot development (WSSA group #8), and microtubule assembly (WSSA group #3). Shoot or root development, or microtubule assembly is slowed or stopped through the disruption of a weed’s plant cell division, a critical function in the growth of a plant. Injury symptoms include stunted growth, crinkled leaves, reduced seedling emergence, and leaf buds that fail to open. One of the more common herbicides in this family is pendamethalin (3). 

Using Herbicide Mode of Action to Your Advantage

In summary, herbicide families are grouped by mode of action. Every herbicidal active ingredient has a site of action and a WSSA classification number. Green industry professionals can benefit from understanding the Mode of Action for each product used in their operations. Labeling an herbicide’s WSSA group number, or Site of Action, is currently voluntary. If a label does not contain this information, it can be accessed through the WSSA, HRAC, and the National Pesticide Information Center.

Herbicide Mode of Action can be used to your advantage. Plan to rotate herbicides from different Sites of Action to prevent resistance in your operation. Further, injury to desirable plants can be diagnosed with an understanding of herbicides in the major Mode of Action groups. 

References:

Baumann, P.A., Dotray, P.A., & Prostko, E.P. (1999).  Herbicides.  How They Work and the Symptoms They Cause.  

Gunsolus, J.L., & Curran, W.S. (2002).  Herbicide Mode of Action and Injury Symptoms.  University of Minnesota Extension Service.  

Classification of Herbicides According to Site of Action.  Herbicide Resistance Action Committee.  

Peterson, D.E., Thompson, C.R., Shoup, D.E., & Olson, B.L.  (2010).  Herbicide Mode of Action, Kansas State University. 

Summary of Herbicide Mechanism of Action According to the Weed Science Society of America (WSSA).