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Herbicide Classification

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Herbicide Classification Terminology

First and foremost it is important to understand the terms associated with herbicide classification.

Mode of Action
is defined as how a particular herbicide acts on a plant. Currently there are eight modes of action. They include, Lipid Synthesis Inhibition, Amino Acid Synthesis Inhibition, Seedling Growth Inhibition, Growth Regulators, Photosynthesis Inhibition, Cell Membrane Disruption, Pigment Inhibition, and for a few herbicides the mode of action is simply listed as “Unclassified” or “Unknown”. Notice that most of these modes of action deal with regulation or inhibition of plant functions. The 'Unclassifed' or 'Unknown' herbicides do not fit into any of the other groups. Fortunately there are very few herbicide mode of actions listed as unknown.

The Site of Action is the biochemical pathway a particular herbicide acts upon in a plant. Currently there are over 20 sites of action. The site of action is listed under each mode of action to minimize confusion. Table 1 lists the more common sites of action recognized by the Weed Science Society of America (WSSA). Table 2 provides many common herbicides, active ingredients, WSSA site of action classification and labeled crops.

Herbicide Family
can be considered a group of herbicides that is named in relation to its chemical similarities. For example, halosulfuron, metsulfuron, and prosulfuron are all members of the Sulfonylurea family. Members of the same herbicide family will have the same mode of action and typically the same site of action. In the example above sulfonylureas have acetolactate synthethase (ALS) which is also called acetohydroxyacid synthase (AHAS), as the site of absorption and the mode of action is amino acid synthesis inhibition.

The Common Name describes the chemical name of the active ingredient of the herbicide. Atrazine is the common name for the product Aatrex 90.

The Trade Name is the name under which a product is marketed. Using the atrazine example above you might purchase the product under the trade name Aatrex.

It should be noted that neither the common name nor the trade name alone can be used to accurately determine mode or site of action. You will need to acquire more information such as chemical family to make this determination.

Site of Absorption
is the location where the herbicide is taken up by the plant. This should not be confused with Site of Action which is the biochemical pathway within the plant where the herbicide acts. In some references the site of absorption may be refereed to as the site of uptake. There are three possibilities for the site of absorption; root, shoot, and foliar. Root absorption is, of course, through the roots of the plants. Shoot absorption refers to uptake by the shoots as it passes through the soil on the way to emergence. Many pre emergence herbicides will have root absorption, shoot absorption or both as their site of uptake. Foliar absorption is through emerged leaves. Foliar absorption herbicides must be applied so that the herbicide is directed to the plant leaves rather than to the soil. If a herbicide has more than one site of absorption listed then typically the sites will be listed in primary order of absorption. For example, a herbicide may be listed as S/R which means that its primary order is shoot absorption and the secondary order is root absorption.

It should also be noted that herbicides within the same chemical families may have different sites of absorption. Do not assume that since a particular herbicide is in a family that it will have the same site of absorption as other members of the family.

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