What Are the Different Types of Herbicides

By - November 6, 2018
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Herbicides are chemicals that are used to kill undesirable plants, such as weeds. The proper use of herbicides has many benefits in saving money, time and labor. But it also can cause problems such as killing non-target plants, and some herbicides, such as Roundup, could be associated with a high risk of some cancers.

Herbicide Types

Herbicides can be classified in several ways: effect, selectivity, persistence, application and action. They also can be divided further into types that are used for organic growing techniques and non-organic growing techniques.


  • Pre-emergent herbicides: Work as they interfere with seed germination. Pre-emergents need to be well-timed per the germination time of the weed that needs to be controlled. But when the timing is correct, they can provide the optimum solution to a weed problem; this type of herbicide is preventative rather than curative.
  • Contact herbicide: Wroks by killing all parts of the plant that they contact. In the majority of cases, these will not kill the whole plant and must be reapplied on a periodic basis. This will eventually drain the energy of the plan and kill it after the herbicide is applied several times.
  • Systemic herbicides: Kill the whole plant over a short period. Conducted throughout the plant in its vascular system. Many types of weeds are resistant to these forms of herbicides. Repeat applications could be in order.
  • Drench herbicides: Applied to soil instead of along with plant application. Most of these last a long time and are inappropriate for using where plants are grown.


Selective herbicides are made to kill certain target plants and leave the plants that are desired unharmed. Herbicides that are used against lawn weeds are ususally selective herbicides. Non-selective herbicides are made to kill any type of plant that they contac, but some plants are resistant to a variety of herbicdes.


Many herbicides have different levels of persistence. This means that some are active for a longer period of time, and others become inactive after they are applied in a short time.


Different herbicides may be applied in several ways. Some are a liquid and are sprayed. Some are granuales and some may be applied via an irrigation system or through fumigation. Some may even be painted directly onto the plants. The majority of herbicides are applied with several methods.


The major mechanisms of action for herbicides are:

  • Dessicators: Work by the removal or water from the plant cells, leading to the death of the plant.
  • Acids and bases: Work like dessicators do by burning plant cells chemically. These are very acidic or alkaline substances.
  • Nutritional controls: Work by shifting nutrient balance by providing too much of one nutrient or too much of another. This usually involves changing the pH of the soil, but sometimes certain nutrients may be used to control certain types of plants.
  • ACCcase inhibitors: Compounds that can kill varieties of grasses. ACCase inhibitors affect the cell membrane production in the grass plant meristems. ACCases of most grasses are sensitive to these types of herbicides.
  • ALS inhibitors: This type of herbicide can slowly starve the affected plans of vital amino acides, which can lead to inhibition of synthesis of DNA.
  • EPSPS inhibitors: This type affects both grasses and dicctos.
  • Synthetic auxin: A form of organic herbicides that were discovered in the 40s after study of plant growth.

Glyphosate Herbicide

Glyphosate, which is the active ingredient in Monsanto’s Roundup, is a broad spectrum systemic herbicide and crop desiccant. It is known as an organophosphorous compound and works by inhibiting key plant enzymes. It is effective in killing weeds, especially the common annual broadleaf weeds and grasses that damage crops. Glyphosate was discovered by a Monsanto herbicide chemist in 1970. It was brought to market in 1974 under the name Roundup.

Glyphosate is effective because it is absorbed through foliage and not as much through roots. It is then transported to growing points of the plants. It inhabits plant enzymes that are involved in synthesizing the amino acids tyrosine, tryptophan and phenylalanine. It is most effective on plants that are actively growing and not very effective as a pre-emergence herbicide. More crops have been engineered genetically to be resistant to glyphosate, such as soybeans, which allows some farmers to use glyphosate as an herbicide post-emergent against many weeds.


  • Glyphosate. (n.d.). Retrieved from https://en.wikipedia.org/wiki/Glyphosate

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