herbicide


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herbicide

(hr`bəsīd'), chemical compound that kills plants or inhibits their normal growth. A herbicide in a particular formulation and application can be described as selective or nonselective. In agriculture, selective herbicides are often used instead of tillage, or in combination with tillage and other agronomic practices, to control weeds without damaging crops. For these no-till or low-till systems, scientists have used biotechnology to develop crop varieties with increased tolerance for herbicides. Nonselective herbicides (e.g., paraquat) toxic to all plants, are used where complete control of plant growth is required.

Contact herbicides kill only the parts of the plant they touch; systemic herbicides are absorbed by foliage or roots and translocated to other parts of the plant. Preemergence herbicides, mixed into the soil, will kill germinating seeds and small seedlings. Postemergence herbicides either hinder photosynthesis or inhibit growth.

Early chemical herbicides were inorganic compounds. Herbicides such as ashes, common salts, and bittern have been used in agriculture since ancient times. Observation in 1896 that Bordeaux mixtureBordeaux mixture
, fungicide consisting of cupric sulfate and lime in water. Its fungicidal activity is associated with the slow formation of copper compounds, the ultimate toxicant being the cupric ion.
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, a fungicide, also provided control of certain weeds, led to the use of copper sulfate as a selective weed killer to control charlock (wild mustard) in cereals. By 1900, solutions of sulfuric acid, iron sulfate, copper nitrate, and ammonium and potassium salts were known to act as selective herbicides; soon thereafter sodium arsenite solutions became the standard herbicides, and they were used in large quantities until about 1960. Other inorganic herbicides include ammonium sulfamate, carbon bisulfide, sodium chlorate, sulfuric acid solutions, and formulations containing borate.

Organic herbicides began to be produced in earnest with dinitrophenol compounds in 1932. A breakthrough occurred in the 1940s with 2,4-D (2,4-dichlorophenoxyacetic acid), a compound similar to plant hormones, which is a highly selective systemic herbicide when used in very small quantities. 2,4-D was quickly adopted to control broad-leaved weeds in corn, sorghum, small grains, and grass pastures, as well as in lawns and other ornamental turf. The phenoxyaliphatic acids and their derivatives, another major group of organic herbicides, succeeded because of their selectivity and ease of translocation. Other groups of organic herbicides include organic arsenicals, substituted amides and ureas, nitrogen heterocyclic acids, phenol derivatives, triazines, and sulfonylureas.

In the 1960s and 1970s, a combination of 2,4-D and 2,4,5-T was widely used in Vietnam as a defoliant under the name Agent OrangeAgent Orange,
herbicide used by U.S. forces during the Vietnam War to expose enemy guerrilla forces in forested areas. Agent Orange contains varying amounts of dioxin. Exposure to the defoliant has been linked with chemical acne, non-Hodgkin's lymphoma, Hodgkin's disease,
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. As a result of questions concerning the possible health effects of the use of Agent Orange, heightened awareness of possible ecological and health dangers attributable to herbicides has resulted in reevaluation of many compounds and has called indiscriminate use into question. Use of the dioxin-containing 2,4,5-T was prohibited in the United States in 1984. In 1975, Mexico, at the urging of the United States government, began spraying fields of marijuana with paraquat, which both eliminated the crop and raised fears of toxic side effects in marijuana users.

Glyphosate, a compound first identified as a herbicide in 1970 and sold beginning in the 1970s (initially only under the tradename Roundup), has been widely used as a broad-spectrum weedkiller because of its apparent relatively low toxicity and tendency to degrade relatively quickly in the environment. Beginning in the 1990s, the use of crop strains that were resistant to its herbicidal effects contributed to the herbicide's much more widespread use and led to the development of so-called superweeds, which have resistance to glyphosate.

See also pesticidepesticide,
biological, physical, or chemical agent used to kill plants or animals that are harmful to people; in practice, the term pesticide is often applied only to chemical agents.
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.

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herbicide

[′her·bə‚sīd]
(materials)
A chemical agent that destroys or inhibits plant growth.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

herbicide

a chemical that destroys plants, esp one used to control weeds
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
At least 30 countries have banned the herbicide while many others are contemplating to prohibit its use.
This reduced the herbicide "workload" and lowered the selection pressure for resistant weeds.
Despite the injuries caused to the plants, a single application of glyphosate or bentazon presented higher selectivity compared to the other post-emergence herbicides, not differing from the treatment with no herbicide application.
A proper application of herbicides adopting recommended weed management methods could substantially increase food production along with saving huge crops worth over Taka 7,000 crore annually, reports BSS.
The ministry had ordered the ban on May 28 after herbicides were suspected in the death of 14 people and hospitalisation of hundreds more in Kratie province earlier that month.
Following the Section 18 Emergency Use Exemption certification in the US, the company's Tough 5EC Herbicide (Pyridate) can now be used for post-emergent control of herbicide resistant weeds in mint in several counties in the US in the states of Idaho, Indiana, Michigan, Oregon, Washington, and Wisconsin.
Another change agent, Emmanuel Okwor who works with the Benue State Agricultural Development Authority (BNARDA) said: 'I used to think taking milk was an antidote to herbicide ingestion but today I have come to realize that milk does not help reduce the effect of herbicides on human health.'
The benefits of applying herbicide safeners as seed treatments are twofold: injury from herbicides is greatly decreased, and the safener is selectively applied to the crop [24].
The task force could not have reached its consensus without mutual trust in the good intentions of all members, and trust in the county's willingness to accept limitations on herbicide use.
An Adjuvant is any compound which is added to herbicide formulation or tank mixture to enhance the efficacy of herbicides.
The third generation of Herbicide Booster is designed with smaller particles to blend quickly with all types of herbicide chemistry.