Phenoxy herbicide

A phenoxy herbicide is any member of a family of chemicals related to the growth hormone indoleacetic acid (IAA). When sprayed on broad-leaf plants they induce rapid, uncontrolled growth, eventually killing them. When sprayed on crops such as wheat or corn, it selectively kills just the broad-leaf plants in a field, the weeds, leaving the crops relatively unaffected. There are a wide variety of phenoxies in use today, further grouped into the phenoxyacetic, phenoxybutyric and phenoxypropionic subtypes, the latter itself containing the aryloxyphenoxypropionic subtype, which contains the greatest number of commercial variants. Chemically they are acids, and are 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 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 does not have this problem, and remains one of the most used herbicides in the world.