Phenoxy herbicides (or "phenoxies") are a family of chemicals related to the growth hormone indoleacetic acid (IAA). When sprayed on broad-leaf plants they induce rapid, uncontrolled growth ("growing to death"). When sprayed on monocotyledonous (grass) crops such as wheat or corn, they selectively kill broad-leaf weeds, leaving the crops relatively unaffected. The wide variety of phenoxies in use today can be grouped into the phenoxyacetic, phenoxybutyric and phenoxypropionic subtypes, the latter containing the aryloxyphenoxypropionic subtype with the greatest number of commercial variants. Chemically, they are carboxylic acids, typically applied in an ester or salt form.
First introduced in 1946, these herbicides were in widespread use in agriculture by the middle of the 1950s. The best known phenoxy herbicides are MCPA, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). 2,4,5-T was later found to be inherently contaminated with the dioxin 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD), and has since been banned in most countries. 2,4-D may also contain dioxin impurities, depending on the production method.
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