Phenoxy herbicides (or "phenoxies") are two families of chemicals that have been developed as commercially important herbicides, widely used in agriculture. They share the part structure of phenoxyacetic acid. Auxins The first group to be discovered act by mimicking the growth hormone indoleacetic acid (IAA). When sprayed on broad-leaf plants they induce rapid, uncontrolled growth ("growing to death"). Thus when applied to monocotyledonous crops such as wheat or corn, they selectively kill broad-leaf weeds, leaving the crops relatively unaffected. Introduced in 1946, these herbicides were in widespread use in agriculture by the middle of the 1950s. The best known phenoxy herbicides are (4-chloro-2-methylphenoxy)acetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Analogues of each of these three compounds, with an extra methyl group attached next to the carboxylic acid, were subsequently commercialised as mecoprop, dichlorprop and fenoprop. The addition of the methyl group creates a chiral centre in these molecules and biological activity is found only in the (2R)-isomer (illustrated for dichlorprop). Other members of this group include 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) and 4-(4-chloro-2-methylphenoxy)butyric acid (MCPB) which act as propesticides for 2,4-D and MCPA respectively: that is, they are converted in plants to these active ingredients. All the auxin herbicides retain activity when applied as salts and esters since these are also capable of producing the parent acid in situ. The use of herbicides in US agriculture is mapped by the US Geological Survey. In 2019, the latest date for which figures are available, 2,4-D was the most used of the auxins. were sprayed that year, compared to of the next most heavily applied, MCPA. The other auxin now used in comparable amounts to 2,4-D is dicamba, where the 2019 figure was . It is a benzoic acid rather than a phenoxyacetic acid whose use has grown rapidly since 2016 as crops genetically modified to be resistant to it have been cultivated. ACCase inhibitors In the 1970s, agrochemical companies were working to develop new herbicides to be complementary to the auxins. The aim was to find materials which would selectively control grass weeds in broad-leaf crops such as cotton and soybean. In 1973, Hoechst AG filed patents on a new class of compound, the aryloxphenoxypropionates, which showed such selectivity and led to the commercialisation of diclofop. Then the Japanese company Ishihara Sangyo Kaisha (ISK) found improved biological activity in an analogue, chlorazifop, which replaced the aryloxy portion of diclofop with