The Eight Modes of Action

Now that you understand the terms the structure of herbicide classification we will now go through a brief overview of the eight modes of actions. 

1. Amino Acid Synthesis Inhibitors.

The amino acid synthesis inhibition mode of action includes herbicides from the following chemical families: sulfonylureas, imidazolinones, triazolopyrimidines, epsp synthetase inhibitors, and the glutamine sythetase inhibitors. The sulfonylureas, imidazolinones, and triazolopyrimidines are also known as ALS or AHAS inhibitors. All of the herbicides within this mode of action act upon specific enzymes to prevent production of amino acids. Amino acids are the “building blocks” for proteins for plant growth and development of a plant. Given the large number of families in the mode of action there are many product names from these herbicide families including, Classic(ALS), Pursuit (ALS) Roundup (EPSP), and Liberty (Glutamine).

Typical injury resulting from the application of an ALS herbicide.

Example of an ALS herbicide, Pursuit, being applied to various plants.

2. Seedling Growth Inhibitors. 

The seedling growth inhibition mode of action is a mode of action that interrupt new plant growth and development. Herbicides within this mode of action must be soil applied and either inhibit root or shoot growth in emerging plants. carbamothiates, acetamides, and the dinitroanilines make up of the herbicide families within this mode of action. Common trade names in this mode of action include Eptam, Dual, Harness, Prowl, and Treflan.

Symptoms of seedling growth inhibitors herbicides.

Example of Dual II Magnum, a seedling growth inhibitor herbicide applied at a 2X rate.

3. Growth Regulators.

Growth regulators are often referred to as auxins since this class of herbicides will mimic the action of auxins within the plant causing rapid and or uncontrolled growth. The site of action is believed to be hormone receptors inside the cell yet the entire site of action is not completely understood. Herbicide families within this mode of action include phenoxies, benzoic acids, carboxylic acids, and the picolinic acids. This is the oldest mode of action of synthetic herbicides since this class includes the product 2,4-D. Trade names of some of the herbicides classified as growth regulators also include Clarity, Stinger, Tordon, MCPA, and Paramount.

Growth regulators commonly cause the type of twisting and epinasty seen in the picture.

Growth regulator herbicides, such as Banvel and 2,4-D, may volatilize and move to non target species as these soybeans.

4. Photosythetic Inhibitors. 

Photosynthetic Inhibitors are one of the next major modes of action that were developed following the growth regulators. Herbicides within the photosynthetic Inhibitor mode of action as their name implies inhibit one of several binding sites in the process of photosynthesis. Without photosynthesis the plant cannot make food. In addition several secondary destructive compounds are produced during the inhibition of photosynthesis and therefore the cause of target plant death is more than simple starvation. Since the herbicides within this mode of action inhibit photosynthesis the herbicides only start working once the plants have emerged and are exposed to light. Families within the mode of action include triazines, uracils, phenylureas, benzothiadiazoles, nitriles, and pyridazines. Common herbicides include Atrazine, Sencor, Hyvar, Karmex, Basagran, and Buctril.

Photosynthetic herbicides produce necrosis symptoms eventually overwhelming susceptible species.

Symptoms from photosynthetic inhibitor herbicides, such as Buctril, will begin with necrosis near the end of the leaf.

5. Lipid Synthesis Inhibitors. 

The lipid synthesis inhibitors typically inhibit the synthesis of plant lipids. If lipids are not produced within the plant then production of cell membranes is unable to proceed and new plant growth is halted. The aryloxyphenoxypropionates and the cyclohexanediones are the two families within this mode of action. Examples of trade names of products within this mode of action include Poast, Assure II, and Select.

Grass plants treated with a lipid synthesis inhibitor herbicides will often show purples and eventual death at the growing point of the plant.

Poast Plus, a lipid synthesis inhibitor herbicide, applied to a variety of plants. Note that no broadleaf plants are affected.

6. Cell Membrane Disrupters. 

Herbicides within the Cell Membrane Disrupter Mode of Action react within the plant to form compounds such as super oxides and hydroxyl radicals which then destroy cell membranes. The herbicides are typically not translocated and the herbicide only effects areas of the plant that it contacts. The herbicide families within the mode of action include: diphenylethers, aryl triazolinones, phenylpthalamides, and bipyridilium. Some common trade names of herbicides include Cobra, Blazer, Authority, Aim, and Gramoxone.

Cell membrane disrupter herbicides are contact herbicides so good spray coverage is very important to ensure plant death.

As their name suggests, cell membrane disrupter herbicides destroy cell membranes within susceptible plants such as this velvetleaf which has been treated with Blazer.

7. Pigment Inhibitors. 

The Pigment inhibitor mode of action works by preventing the production of compounds that protect the plant from chlorophyll destruction. Instead of being green in color plant tissue turns white. Herbicides within this mode of action are typically preemergence treatments however a few have postemergence activity as well. Isoxazolidinones, isoxazoles, and pyridazinones make up the chemical families in this mode of action. Common trade names of pigment inhibitor herbicides include: Balance, Callisto and Command.

Pigment inhibitor herbicides allow the destruction of chlorophyll which results in plants exhibiting white tissue.

This white morningglory plant is the result of Callisto being applied as a pre-emergent treatment.

8. Unknown. 

A few herbicides mode of action are classified as 'unknown'. This simply means that we do not understand the mode of action or it has not been classified. MSMA and Nortron are the most common herbicides classified as unknown.