Triclopyr
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Triclopyr, also Trichlopyr,<ref>Template:Cite web</ref> formally [(3,5,6-trichloropyridin-2-yl)oxy]acetic acid,<ref name=ChEBI>Template:Cite web</ref> is an organic compound in the pyridine family of herbicides that is used as a systemic foliar type of herbicide<ref name=Ullmann>Template:Cite book</ref><ref name=ChEBI/> and, secondarily, as a fungicide.<ref name="SoyResearch">Template:Cite book</ref> Triclopyr is an acetic acid derivative, and so a monocarboxylic acid, with a pyridyloxy substituent: specifically, a pyridin-2-yloxy group substituted by chloro groups at positions 3, 5, and 6.<ref name=ChEBI/> It functions as an herbicide by mimicking the action of the plant growth hormone auxin.<ref name="WDNR 2022"/>Template:Better source
Structure and chemistry
Template:Expand section Triclopyr is an acetic acid derivative, and so a monocarboxylic acid, with a pyridyloxy substiuent: specifically, a pyridin-2-yloxy group substituted by chloro groups at positions 3, 5, and 6.<ref name=ChEBI/>
History
Triclopyr triethylammonium (TEA) was first registered in 1979 in the United States for use as an herbicide in non-crop areas and forestry to control broadleaf weeds and woody plants. A year later, in 1980, a formulation containing triclopyr butoxyethyl ester (BEE) was registered for similar applications. Both formulations expanded their usage to turf sites in 1984, and in 1985, triclopyr BEE was specifically registered for use on rangeland and permanent grass pastures. In 1995, the triclopyr TEA formulation received registration for use on rice crops to manage broadleaf weed species.<ref>Template:Cite web</ref>
Triclopyr's usage in the US in 2018 was Template:Convert, and has held at roughly this level since 1998, by USGS estimates. It is mostly used on pasture and hay, with a small but consistent use on rice.<ref>Template:Cite web</ref>
Uses
As presented by the Wisconsin Department of Natural resources, triclopyr is a systemic herbicide and is believed selective in its control of dicotyledonous (broadleaf) weeds and woody plants,<ref name="WDNR 2022"/> while leaving monocotyledonous plants (such as grasses, bulbs, and conifers) largely unaffected.<ref name=Ullmann/> The fact sheet from this Wisconsin agency goes on to state, "It is a WSSA [Weed Science Society of America] Group 4 herbicide, meaning that the mechanism of action is by mimicking the plant growth hormone auxin",<ref name="WDNR 2022"/> and that "[p]lant growth becomes abnormal and twisted following treatment, and plants decompose within one to two weeks after application".<ref name="WDNR 2022">Template:Cite webTemplate:Better source</ref> The guidance goes on to state that
Triclopyr needs to be applied to plants that are actively growing. If there is flowing water at a treated site, higher concentrations or a repeated application may be required.<ref name="WDNR 2022"/>
Triclopyr is a widely used herbicide with active registrations in Australia,<ref>Template:Cite web</ref> the United States, Brazil, Canada, Morocco, Paraguay, South Africa and Southeast Asia.Template:Cn In the United States, it is sold under the trade names Garlon, Remedy, Turflon, Weed-B-Gon, Brush-B-Gon, and RoundUp Brush Killer,<ref name="Purdue 2024">Template:Cite magazine</ref> and in the United Kingdom as SBK Brushwood Killer.Template:Cn It is also a major ingredient in Confront.Template:Cn In addition to weed control, triclopyr has also been documented for use in controlling rust fungus on soybean crops.<ref name="SoyResearch" />
Environmental effects
Triclopyr, including its ester and amine salt forms, is rapidly converted to the triclopyr acid form in the environment. The acid form is largely water-soluble, while the ester form exhibits lower solubility. Triclopyr has a low vapor pressure, indicating a limited tendency to volatilize into the atmosphere. In aquatic environments, triclopyr degradation is accelerated by light, with a reported half-life of approximately 1 day under light conditions. Without light, it is more persistent in water, with a half-life of around 142 days.<ref name="Strid 2018">Template:Cite web</ref>
Triclopyr undergoes relatively rapid breakdown in soil, primarily through microbial activity. Soil half-lives typically range from 8 to 46 days, with longer persistence observed in deeper, less oxygenated soils. Although triclopyr is considered mobile in soil, studies have indicated limited movement below depths of 15 to 90 centimeters. Its movement in soil is influenced by factors such as organic matter content and rainfall. As a systemic herbicide, triclopyr is absorbed by plants through both leaves and roots and tends to accumulate in active growth regions. The rate of breakdown within plants varies widely depending on the species; reported half-lives in plants range from 3 to 24 days, with some plants like barley and wheat showing more rapid degradation.<ref name="Strid 2018"/>
The compound is slightly toxic to ducks (LD50 = 1698 mg/kg) and quail (LD50 = 3000 mg/kg).<ref>EXTOXNET (Extension Toxicology Network), Oregon State University</ref> It has been found nontoxic to bees<ref name="Strid 2018"/> and marginally toxic to fish (rainbow trout LC50 (96 hr) = 117 ppm).<ref name="Environmental Fate Of Triclopyr">Environmental Fate Of Triclopyr, Carissa Ganapathy, Environmental Monitoring & Pest Management Branch Department of Pesticide Regulation Sacramento, CA (dead link 23 October 2023)</ref>
Garlon's fact sheet for a triclopyr ester—versus the triclopyr amine salt per se—indicates that further derivative to be highly toxic to fish, aquatic plants, and aquatic invertebrates, and so never used in waterways, wetlands, or other sensitive habitats.<ref>Template:Cite web</ref>