herbicide

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Related to Contact herbicide: Systemic herbicide

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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.

The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/

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 ?
They are capable of controlling perennial plants and may be slower acting but ultimately more effective than contact herbicides (Howard, 1992).
Rely is a preferred contact herbicide for mid-season and post-harvest spraying after pre-emergent herbicides have become ineffective and can't be applied again, but it is in short supply because of demand in the southeast for use with cotton and soybeans fighting glyphosate-resistant weeds.
A few weeks later, pull and hoe the sprouts or apply a contact herbicide such as glyphosate.
In two to three weeks, weed seedlings can then be easily pulled, hoed, or killed with a contact herbicide, such as glyphosate, without harming the bulbs.
In-row weed control requires tillage, propane for weed burn down or multiple applications of a contact herbicide. All increase fuel consumption compared to conventional weed control using an ATV and a synthetic herbicide.
Contact herbicides applied at the beginning of the sunflower cycle were likely to suppress volunteer soybean plants and allow the recovery of the sunflower crop (Brighenti, 2015).