2,4-Dichlorophenoxyacetic acid (usually called 2,4-D) is an organic compound with the chemical formula C8H6Cl2O2 . It is a systemic herbicide that selectively kills most broadleaf weeds by causing uncontrolled growth in them, but leaves most grasses such as cereals, lawn turf, and grassland relatively unaffected. 2,4-D is one of the oldest and most widely available herbicides and defoliants in the world, having been commercially available since 1945, and is now produced by many chemical companies since the patent on it has long since expired. It can be found in numerous commercial lawn herbicide mixtures and is widely used as a weed killer on cereal crops, pastures, and orchards. Over 1,500 herbicide products contain 2,4-D as an active ingredient.
2,4-D is a member of the phenoxy family of herbicides. 2,4-D is manufactured from chloroacetic acid and 2,4-dichlorophenol, which is itself
produced by chlorination of phenol. Alternatively, it is produced by the chlorination of phenoxyacetic acid. The production processes create several contaminants including di-, tri-, and tetrachlorodibenzo-p-dioxin isomers and N-nitrosamines, as well as monochlorophenol.
2,4-D is primarily used as a selective herbicide that kills many terrestrial and aquatic broadleaf weeds, but not grasses. It acts by mimicking the action of the plant growth hormone auxin, which results in uncontrolled growth and eventually death in susceptible plants.
In agriculture, 2,4-D is the cheapest way for farmers to control winter annual weeds by spraying in the fall, often at the lowest recommended rate. This is particularly effective before planting beans, peas, lentils, and chickpeas. In domestic lawn and garden maintenance, 2,4-D is commonly used for weeding control in lawns and other turfs. It is used to kill unwanted weeds such as dandelions, plantain, clover, and chickweed. In forestry, it is used for stump treatment, trunk injection, and selective control of brush in conifer forests. Along roadways, railways, and power lines, it is used to control weeds and brush which might interfere with safe operation and damage equipment. It is often used by government agencies to control the spread of invasive, noxious, and non-native weed species and prevent them from crowding out native species, and to control many poisonous weeds such as poison ivy and poison oak. 2,4 D has been used in laboratories for plant research as a supplement in plant cell culture media such as MS medium since at least 1962. 2,4-D is used in plant cell cultures as a dedifferentiation (callus induction) hormone. It is classified as an auxin plant hormone derivative.