Triclopyr, also Trichlopyr,[ and, secondarily, as a fungicide.][ It biological mechanism as an herbicide lies in its mimicking the action of the plant hormone, auxin.]
Structure and chemistry
Triclopyr is an acetic acid derivative, and so a monocarboxylic acid, with a substiuent, specifically, a pyridin-2-yloxy group substituted by chloro groups at positions 3, 5 and 6.[
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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.
Triclopyr's usage in the US in 2018 was , 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.
Uses
As presented by the Wisconsin Department of Natural resources, triclopyr is a systemic herbicide and is believed selective in its control of Dicotyledon and woody plants,[ The fact sheet from this Wisconsin agency goes on to state, "It is a WSSA Weed Group 4 herbicide, meaning that the mechanism of action is by mimicking the plant growth hormone auxin",]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.
Triclopyr is a widely used herbicide with active registrations in Australia,
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.
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.
The compound is slightly toxic to ducks (LD50 = 1698 mg/kg) and quail (LD50 = 3000 mg/kg).[ EXTOXNET (Extension Toxicology Network), Oregon State University] It has been found nontoxic to bees[ Environmental Fate Of Triclopyr, Carissa Ganapathy, Environmental Monitoring & Pest Management Branch Department of Pesticide Regulation Sacramento, CA (dead link 23 October 2023)]
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.
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