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Herbicides, also referred to as weedkillers, are Pesticides used to kill unwanted plants. Herbicides account for about 69% of all pesticide use in the United States, where they are used on around 90 million hectares, or around half of the country's farmlands. Over 50% of this herbicide use is for corn, and 25% is for soy. Herbicides are also used both privately and publicly to control weeds in gardens and parks, on school grounds and sports fields, along roads, sidewalks, and fences, etc. The herbicide Agent Orange was used extensively in the Vietnam War to defoliate broadleaf plants. ("Broadleaf" refers to plants that have leaves that are not needles. Grasses, while technically having broad leaves, are typically categorized separately.)


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Prior to the 1930s, herbicides were non-specific in what they killed and also very toxic to humans. Advances in synthetic chemistry in the 1930s and 1940s led to the creation of many new Insecticides and herbicides. These new chemicals were more specific in what they targeted but still far from perfect.

Herbicide use was not limited to agriculture. The herbicide Agent Orange was used extensively in The Vietnam War to defoliate broadleaf plants.


Herbicides can be grouped by activity, timing of application, mechanism of action, or type of vegetation controlled.

a) By activity:
Contact herbicides destroy only plant tissues that come into direct contact with the herbicide. Generally, these are the fastest acting herbicides. They are less effective on perennial plants, which are able to regrow from roots or tubers.

Systemic herbicides are translocated through the plant, either from foliar application down to the roots, or from soil application up to the leaves. They can destroy a greater amount of plant tissue than contact herbicides.

b) By timing of application:
Pre-emergent herbicides are applied to the soil before the weed emerges to prevent germination or early growth of weed seeds.

Post-emergent herbicides are applied after the weed has emerged.

c) By mechanism of action:

Their classification by mechanism of action (MOA) indicates the first enzyme, protein, or biochemical step that the herbicide affects in the plant.

ACCase inhibitors are grass killers that inhibit the first step of lipid synthesis by inhibiting the compound Acetyl coenzyme A carboxylase (ACCase). ACCase inhibitors affect cell membrane production in the meristems (growth points) of the grasses. The ACCases of grasses are sensitive to these herbicides, whereas the ACCases of dicot plants are not. (All broadleaf plants can be separated into two groups, monocots and dicots. Grasses are monocots.)

ALS inhibitors slowly starve plants by inhibiting the synthesis of branched-chain amino acids (valine, leucine, and isoleucine), which eventually inhibits DNA synthesis. ALS refers to the acetolactate synthase enzyme (also known as acetohydroxyacid synthase, or AHAS) which is required for synthesis of the amino acids. This family of herbicides affects grasses and dicots alike and includes these subtypes: sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and sulfonylamino carbonyl triazolinones (SCTs).

EPSPS inhibitors inhibit an enzyme (enolpyruvylshikimate 3-phosphate synthase) used in the synthesis of the amino acids tryptophan, phenylalanine, and tyrosine. They affect grasses and dicots alike. Glyphosate (Roundup) is a systemic EPSPS inhibitor that is inactivated by soil contact.

Synthetic auxins inaugurated the era of organic herbicides. They were discovered in the 1940s after a long study of the plant growth regulator auxin. Synthetic auxins mimic this plant hormone. They have several points of action on the cell membrane, and are effective in the control of dicot plants. 2,4-D is a synthetic auxin herbicide.

Photosystem II inhibitors disrupt part of the photosynthetic process. They reduce electron flow from water to NADPH2+ at the photochemical step in photosynthesis. They bind to the Qb site on the D2 protein, and prevent quinone from binding to this site. This cause electrons to accumulate on chlorophyll molecules. As a consequence, oxidation reactions in excess of those normally tolerated by the cell occur, and the plant dies. The triazine herbicides (including atrazine) are PSII inhibitors.

d) By type of vegetation controlled

Selective herbicides are effective on certain kinds of plants, such as monocots (grasses) or dicots (broadleaf plants).

Nonselective herbicides damage most plants regardless of the type.


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