Herbicides

Written by Rachel Frost, Montana State University

Herbicides that interfere with and/or disrupt the biochemical or physiological processes unique to plants. Herbicides typically decreases the growth, competitiveness, and/or seed production of unwanted plant species while providing opportunities for desired species to increase in number, size and productivity. Herbicides, therefore, are valuable tools for controlling unwanted plants on rangelands.

When properly applied, herbicides can provide a window of opportunity for desired plants by removing or suppressing populations of weeds. However, using herbicides alone to control weeds seldom results in successful long-term control. Once weed species are removed from a site, the area must be revegetated with desired species that can competitively exclude the potential weed species. Herbicides are most valuable when they are one component of an integrated weed management plan that focuses on replacing of weeds with desired species, proper grazing management, and management actions that reduce the risk of new infestations.


How Herbicides Work

Written by Rachel Frost, Montana State University

Application. Most herbicides are applied either pre-emergence, before the weeds emerge from the soil and begin to grow, or post-emergence, when weeds are already growing and easily located. Both pre- and post-emergent herbicides are used on rangelands. Pre-emergent herbicides have a high degree of soil activity and are readily absorbed by the roots of both seedlings and mature plants. The target plants are killed shortly after they germinate and/or shoots emerge from the soil. Pre-emergence herbicides are effective in controlling annual weeds, such as cheatgrass, and may be applied following disturbances like wildfire to prevent weed establishment from seeds.

Formulation. A herbicide formulation is how a particular herbicide is packaged for distribution. The formulation includes both active ingredients — the chemical that harms or kills the plant — and inert ingredients such as solvents that enable them to penetrate leaf tissue. The most common formulation of rangeland herbicides is a concentrated liquid that can be diluted with water and sprayed on the target species. Water is the most commonly used carrier of herbicides for rangeland application. It is cheap, universally available, and works with a wide variety of herbicides. However, hard water — water with high levels of calcium and magnesium salts — can decrease the activity of certain herbicides with an ionic charge. A few rangeland herbicides are formulated as dry granular material or pellets that are applied directly to the soil surface without dilution in water. Because it is not diluted prior to application, the active ingredient in dry formulations is much less concentrated. These herbicides require precipitation to move the active ingredient into the soil to the root system, where it is absorbed and translocated to the plant's growing points.

Selectivity. The selectivity of an herbicide is determined by the plant's ability to metabolize the active ingredient and render it harmless. The different metabolic processes in plants are capable of inactivating certain herbicides and rendering them harmless. For example, grass species are capable of metabolizing phenoxy herbicides, while broadleaf plants are not. Phenoxy herbicides, therefore, kill broadleaf plants but have no effect on most mature grass plants. Seedlings are the growth stage most susceptible to chemical control. An herbicide that does not harm mature plants may kill most of the seedlings of the same species. It is important that anyone applying an herbicide read the herbicide's product label before they apply the chemical. The product label will inform the applicator of the herbicide's level of selectivity and how that may change with plant maturity or the dosage applied. Understanding an herbicide's selectivity is important because land managers often want to remove one or more species without adversely affecting the non-target species.

Translocation. Once an herbicide has been absorbed by a plant, the movement of the active ingredient throughout the plant is referred to as translocation. Systemic herbicides are absorbed by the plant's roots and/or foliage and are translocated to distant parts of the plant, including root buds, growing points (meristematic tissue), and other reproductive structures. The active ingredient typically accumulates in these critical growth areas and interrupts the important metabolic processes that keep the plant alive. Systemic herbicides are very effective at controlling perennial weeds that regrow from buds on the roots or root crowns. To kill perennial plants that resprout from belowground buds, all of the buds have to be killed. Systemic herbicides that are translocated through the phloem are most commonly used on rangeland. Contact herbicides have very limited movement within the plant and kill only the tissue that comes in direct contact with the herbicide. Although contact herbicides can effectively top-kill a perennial plant, the herbicide's inability to be translocated to the root system allows the plant to regrow the following year. Contact herbicides are most effective against annual plants.


Successful Weed Management with Herbicides

Written by Rachel Frost, Montana State University

Proper attention to the following three basic principles will improve the effectiveness of herbicides and decrease potential negative impacts to non-target species, the environment, and the applicator:

  • Choose the right herbicide for the job - Herbicides vary in the way that they affect plants and in the type of plants that they affect. Non-selective herbicides kill or suppress all vegetation, while selective herbicides kill some plants but not others. For example, a broadleaf spectrum herbicide like 2,4-D kills only broadleaf forbs and shrubs and does not harm grasses and sedges.
  • Apply the herbicide at the right time - Herbicides need to be applied at the correct stage of plant growth to maximize effectiveness. For example, annual weeds should be treated before flowering to prevent seed set. Movement of the herbicide through the plant's system to the roots is essential for perennial weed control, so herbicides are most effective when applied to perennial plants when they are moving carbohydrates to the roots.
  • Use the proper application technique - Herbicides can be mixed in a variety of formulations, including liquids, powders, or pellets. The label is a legal document that provides information on the proper use and application of herbicides. Labels provide detailed information on the correct formulation, rate of application, recommended carriers or additives, and safety precautions for that specific herbicide.
    • Additional site characteristics such as soil type, slope, and the existing vegetation — both target and non-target plants — should also be considered when selecting the herbicide and planning the application process.
    • Always read and follow the herbicide label directions.
    • Check with your local weed professional or Cooperative Extension agent for help in selecting the proper herbicide and application procedure for your target species. Remember to calibrate your sprayer to ensure accuracy in application rates and to save money.

Herbicide Mode of Action

Written by Rachel Frost, Montana State University

The mechanism by which herbicides actually kill plants is known as the herbicides’ mode of action. Each mode of action focuses on a specific site of action, which is usually a single enzyme or enzyme pathway that is essential for plant growth. Herbicides kill plants by inhibiting or affecting these essential enzymes or pathways.

Growth-regulating herbicides. The most widely used herbicides in rangelands are growth-regulating herbicides. Growth-regulating herbicides upset the normal hormonal balance that regulates processes such as cell division, cell enlargement, protein synthesis, and respiration. Although usually applied to the foliage, this group of versatile herbicides is also effective in the soil. This increases the effectiveness of the herbicide as any of the chemical that does not land on the foliage can be percolated into the soil with rain and taken up by the weed roots. Growth-regulating herbicides can be classified as phenoxy herbicides, benzoic acids, and carboxylic acids.

Photosynthesis inhibitors. Photosynthesis inhibitors do just what their name implies: They inhibit photosynthesis, preventing plants from converting light energy from the sun into sugars used for food. Photosynthesis herbicides are used primarily to control broadleaf plants but can be marginally effective against annual grasses.

Amino acid synthesis inhibitors. There are two main types of amino acid synthesis inhibitor herbicides: selective and non-selective. Selective amino acid synthesis inhibitors control both broadleaf plants and grasses, have both soil and foliar activity, and are virtually non-toxic to mammals and most non-vegetative life forms. Amino acid synthesis inhibitors kill the plant by binding to a specific enzyme and preventing the plant from synthesizing essential amino acids. Non-selective amino acid inhibitors, such as Roundup, control a broad range of plants, including grasses, sedges, and forbs. They bind tightly to soil clay and organic matter and do not move through the soil profile.

Written by Rachel Frost, Montana State University

What happens to herbicides after they have been applied to rangeland? Do they remain in the soil, eventually accumulating to hazardous levels? Or do they move into the environment, causing off-site problems? In reality, most herbicides do not survive in the soil for very long, nor do they move very far through the soil from one area to another. The environmental persistence of a herbicide depends on the rate of application, the method of application, soil type, weather, and characteristics of the chemical. Still, herbicides are subject to the same biological processes of decomposition as any other compound.