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

Wheat

Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...

 or corn

Maize

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

, 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

Ester

Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 or salt

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 form.

First introduced in 1946, these herbicides were in widespread use in agriculture

Agriculture

Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 by the middle of the 1950s. The best known phenoxy herbicides are 2,4-dichlorophenoxyacetic acid

2,4-Dichlorophenoxyacetic acid

2,4-Dichlorophenoxyacetic acid is a common systemic pesticide/herbicide used in the control of broadleaf weeds. It is the most widely used herbicide in the world, and the third most commonly used in North America...

 (2,4-D) and 2,4,5-trichlorophenoxyacetic acid

2,4,5-Trichlorophenoxyacetic acid

2,4,5-Trichlorophenoxyacetic acid , a synthetic auxin, is a chlorophenoxy acetic acid herbicide used to defoliate broad-leafed plants. It was developed in the late 1940s and was widely used in the agricultural industry until being phased out, starting in the late 1970s due to toxicity concerns....

 (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.