2,4-Dichlorophenoxyacetic acid, a phenoxy herbicide
A phenoxy herbicide is any member of a family of chemicals related to the growth hormoneindoleacetic 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.