a pyridine ring containing the same two chlorine substituents. This area of research became very competitive and within three weeks of one another in 1977 ISK, Dow Chemicals and Imperial Chemical Industries (ICI) all filed patents covering another group of analogues, with a trifluoromethyl (CF3) group in place of one of the chlorine atoms in the pyridine. Subsequently, ISK and ICI cross-licensed their intellectual property and first marketed fluazifop as its butyl ester in 1981 under the brand name Fusilade while Dow marketed haloxyfop as its methyl ester. All these compounds have an additional oxygen-linked aromatic group in the para position of the phenyl ring with its OCH(CH3)COOH group and as a class are called "fops", referring to their common fenoxy-phenoxy [sic] feature. This group of herbicides acts by inhibiting plant acetyl-CoA carboxylase (ACCase), a completely different mechanism of action to that of the auxins. Their selectivity for grasses arises because they target the plastid isoform of the enzyme present only in these species, making them ineffective on broad-leaf weeds and other organisms including mammals. When applied as an ester, metabolism in the target plant leads to the parent acid which is responsible for the herbicidal action. It is a coincidence that it is the (2R) stereoisomer which binds to plant ACCase, just as that isomer is responsible for the activity of dichlorprop as an auxin. Salts and esters of this class of herbicide are active owing to their ability to metabolise to the corresponding parent acid. For example, fenoxaprop-P ethyl was introduced by Bayer Crop Science and quizalofop-P ethyl by Nissan Chemical Corporation, both in 1989. In 1990, Dow introduced cyhalofop-P butyl for the control of weeds in rice. Fluazifop-P butyl still has significant use in the USA. were applied in 2018 — almost exclusively in soyabean. The "P" in the name of these materials refers to their use now as single enantiomers. References Herbicides