CEU Series: 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.

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.

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.

Knowing the science behind your chemistries can help maximize a weed management plan. Earn one Core CEU credit. Take the Series 44 test now.