Triclopyr: Uses, Health Effects, Safety and Regulation

Triclopyr is a pyridine herbicide used to control woody and broadleaf plants. It is used along rights-of-way, in forests, on industrial lands and on grasslands. Its oral LD50 in rats is 630 to 729 mg/kg. EPA classified it as Group D, not classifiable as to human carcinogenicity. It is slightly to practically non-toxic to birds and practically non-toxic to honey bees. In the environment it breaks down mainly by photodegradation in water and microbial degradation in soil. It is registered with the US EPA, and the EFSA peer review for the EU renewal concluded in 2024. Food and water add 49 percent of the reference dose for children, and aquatic-labeled products are a safer start.

What Is Triclopyr?

Triclopyr is a pyridine herbicide whose parent acid has the chemical name 3,5,6-trichloro-2-pyridyloxyacetic acid. Triclopyr is sold mainly as a triethylamine salt (TEA) or as a butoxyethyl ester (BEE) of that parent compound. The TEA form holds 44.4 percent active ingredient, and the BEE form holds 97.1 percent. The TEA form is corrosive to the eyes of rabbits, while the BEE form causes only minimal eye irritation.

The National Pesticide Information Center publishes the technical fact sheet behind these forms. It is a US pesticide information service sponsored by Oregon State University and EPA. Products that contain triclopyr often hold other herbicides such as clopyralid and 2,4-D. The phenoxy herbicide 2,4-D is often sold in mixtures with triclopyr.

How Does Triclopyr Kill Weeds?

Triclopyr kills weeds by mimicking the effects of plant hormones called auxins. Triclopyr is a selective systemic herbicide. Plant roots or foliage take it up. Triclopyr then moves readily throughout the plant. Broadleaf plants respond to this hormone-like action. Triclopyr alone is ineffective against shrub honeysuckles.

What Is Triclopyr Used For?

Triclopyr is used to control woody and broadleaf plants along rights-of-way, in forests, on industrial lands and on grasslands. Parklands count among its use sites. Its products come as soluble concentrates, emulsifiable concentrates, liquids, granulars, wettable powders and pellets. The TEA form was first registered in 1979 for non-crop areas and forestry. Both forms were registered for turf sites in 1984.

Garlon is a brand name of triclopyr herbicide products. Turflon, Pathfinder, Brush-B-Gon, Confront and Crossbow carry triclopyr too. Access is a triclopyr brand that may be restricted use. Labels write application rates in lb ae/A, which means pounds of acid equivalent per acre.

How Toxic Is Triclopyr to People?

Triclopyr has an oral LD50 of 630 to 729 mg/kg in rats, which sets how toxic it is to people. Triclopyr TEA has an oral LD50 of 1847 mg/kg in rats, and the BEE form has 830 mg/kg. The dermal LD50 of the technical material in rabbits is greater than 2000 mg/kg. EPA placed technical triclopyr acid in Toxicity Category III for oral and dermal effects. The acute inhalation LC50 of the TEA form in rats is greater than 2.6 mg/L over 4 hours.

Homeowner exposure to triclopyr is expected to be minimal because of its low dermal and inhalation toxicity. More than 80 percent of the triclopyr ingested by six human volunteers appeared unchanged in the urine within 48 hours. Acceptable studies were not available for several endpoints of the technical acid. These endpoints are acute inhalation, primary eye irritation, primary dermal irritation and dermal sensitization.

A 2010 study by Yeon Y Kyong, titled Severe systemic intoxication following triclopyr-TEA ingestion, described a poisoned patient. Triclopyr ingestion caused metabolic acidosis and coma with cardiovascular impairment. The patient was extubated 57 h after ingestion and recovered completely.

A study in Clinical toxicology from Soonchunhyang University Cheonan Hospital followed 17 patients who ingested auxin herbicides, including triclopyr. One death followed ingestion of 500 mL of a mecoprop product. The other 16 patients recovered, according to Jae-Seok Park in 2011.

Does Triclopyr Cause Kidney or Liver Damage?

It depends. The dose decides whether triclopyr can cause kidney and liver damage in animals. Organs affected by exposure to triclopyr include the kidneys and liver. Rats fed 3 to 30 mg/kg/day for long periods showed no ill effects.

A diet with 20 mg/kg of triclopyr for 13 weeks degenerates the proximal tubules in rat kidneys. Male rats fed 100 mg/kg/day had decreased liver and body weight and increased kidney weight. Female kidney weights and male liver weights rose in beagle dogs fed 20 mg/kg body weight over 228 days.

Does Triclopyr Affect Reproduction or Development?

It depends. The dose decides whether triclopyr affects development, which happens only at high animal doses. No data was found on the effects of triclopyr on human reproduction or development.

The developmental Lowest Observable Effects Level (LOEL) for the BEE form is 100 mg/kg in rabbits. The developmental No Observable Effect Level is 30 mg/kg. Rats given triclopyr during pregnancy show a decrease in mean fetal body weight and more fetal and litter skeletal anomalies. A two-generation rat study found a NOEL of 5.0 mg/kg/day. A three-generation rat study at 3, 10 and 30 mg/kg/day showed no impact on fertility rates. EPA finds no special sensitivity of young animals to triclopyr residues.

Is Triclopyr Carcinogenic?

No. EPA classified triclopyr as Group D, not classifiable as to human carcinogenicity. The EFSA peer review does not support classifying triclopyr as Carc. Cat 2, because a clear conclusion on carcinogenicity based on mammary gland findings cannot be reached.

Mammary gland adenocarcinomas rose significantly in female mice and rats fed triclopyr at 36 mg/kg/day for 2 years. EPA’s rating rested on mammary tumors in female rats and mice and on adrenal pheochromocytomas in male rats. EPA considered these a marginal response. Rats and mice fed 3 to 30 mg/kg/day for 2 years showed no carcinogenic response. Triclopyr is nonmutagenic in bacterial and cytogenetic assay systems.

How Toxic Is Triclopyr to Fish, Birds and Bees?

Triclopyr is slightly to practically non-toxic to birds and practically non-toxic to honey bees. The LD50 of the TEA form in mallard ducks is 2055 mg/kg. Triclopyr has an acute contact LD50 greater than 100 µg/bee for honey bees. The 96-hour LC50 of triclopyr in rainbow trout is 117 mg/L.

The BEE form has a 96-hour LC50 of 0.1 to 10 ppm for bluegill, rainbow trout and fathead minnow. The TEA form is practically non-toxic to the same fish, with an LC50 greater than 100 ppm. The BEE form is slightly to moderately toxic to Daphnia magna, with an LC50 of 1.7 to 12 mg/L. The metabolite TCP is moderately toxic to fish, including several species of salmon.

LC50 values are expressed as mg of chemical per liter of water. The risk assessment for aquatic organisms in the EU could not be finalised.

Is Triclopyr Toxic to Fish Embryos?

Yes. Triclopyr has a 96-hour LC50 of 87.46 mg/L in zebrafish embryos, and sublethal concentrations harm fish embryos too. A 2024 study by Ítalo Bertoni in Toxics reported an LC50 of 87.46 mg/L (341.01 µM) for triclopyr in zebrafish.

Zebrafish embryos exposed to 50 µM triclopyr showed a 24 percent delay in hatching rate between 72 and 144 hours. Concentrations of 0.5, 1, 5, 10 and 50 µM delayed swim bladder development at 96 hours post-fertilization. Larvae treated with 5 and 10 µM swam shorter total distances during the second dark period.

Triclopyr was tested at 3 sublethal concentrations chosen close to environmental detection levels. The zebrafish embryo acute toxicity test followed the Organization for Economic Co-operation and Development (OECD) Guideline for Testing Chemicals. Juvenile salmonids show LC50 values of 5.3 to 9.7 mg/L.

Researchers from the Department of Pathology at São Paulo State University (Unesp) published a 2026 study. It reported that triclopyr induced bradycardia at all tested concentrations after 48 h of development.

How Does Triclopyr Break Down in the Environment?

Triclopyr breaks down in the environment mainly by photodegradation in water and by microbial degradation in soil. Photodegradation is the predominant pathway for triclopyr in water. Microbial degradation turns triclopyr in soil into the major degradate 3,5,6-trichloro-2-pyridinol, known as TCP. TCP is the main breakdown product of triclopyr and of chlorpyrifos.

Triclopyr acid is somewhat persistent and mobile. Its TCP degradate is both persistent and mobile. In laboratory sediment-water incubations, triclopyr showed moderate persistence and formed the major metabolites. Triclopyr-butotyl showed very low persistence in the same incubations.

How Long Does Triclopyr Persist in Soil and Water?

Triclopyr can persist in soil for 46 days on average and in water for 2.8 to 14.1 hours. The soil range is 30 to 90 days, depending on soil type. A half-life is the time required for half of the compound to degrade. NPIC reports a soil half-life range of 1.1 to 90 days.

Water half-lives of 2.8 to 14.1 hours depend on season and depth of water. NPIC gives 1 to 10 days instead, with sunlight as the main cause of loss and turbidity as the variable. The two sources disagree on water half-life.

TCP has a soil half-life of 30 to 90 days, similar to triclopyr. Longer half-lives may occur in cold or arid conditions. In soil, both forms of triclopyr degrade into several intermediates before ultimately becoming carbon dioxide.

Can Triclopyr Leach Into Groundwater?

It depends. Soil type and label compliance decide whether triclopyr can leach. Triclopyr has the potential to leach into groundwater because it is not strongly adsorbed to soil particles. Batch adsorption studies showed very high to high mobility in soil.

FOCUS groundwater scenarios are EU standard models that estimate groundwater exposure from pesticides. The EU modelling found low groundwater exposure above the 0.1 µg/L drinking water limit in all 9 FOCUS scenarios. EPA requires a label statement warning users that triclopyr can leach to ground water in certain situations.

How Is Triclopyr Regulated in the United States?

Triclopyr is regulated in the United States through registration with the US Environmental Protection Agency. EPA is the federal agency that registers pesticides and reviews their risks. EPA first registered triclopyr TEA in 1979. EPA must register all pesticides sold in the United States. Registration rests on studies showing safe use without unreasonable risks.

The Reregistration Eligibility Decision is EPA’s document that sets whether a pesticide stays registered, and on what conditions. EPA’s decision for triclopyr covers triclopyr acid, the TEA salt and the BEE ester. The Food Quality Protection Act of 1996 requires EPA to weigh extra risk to infants and children from pesticide residues. Some products, such as Access, may be classified as Restricted Use, which only certified applicators may buy and use.

What Restrictions Apply to Triclopyr Products?

The restrictions that apply to triclopyr products are label limits on rates, grazing, harvest timing and warnings. Triclopyr labels set five key restrictions:

• Pasture and rangeland rate limit: The maximum rate on pasture, rangeland and other grazed sites is 1 lb acid equivalent per acre per year. This is the lowest rate listed.
• Forestry rate limit: The maximum rate for forestry applications is 6 lb acid equivalent per acre. Rates are written in lb acid equivalent per acre (lb ae/A).
• Grazing restriction: Labels restrict grazing of lactating dairy animals until the following season. Treated pasture stays off limits to these animals until then.
• Grass hay interval: Labels must specify a 14 day pre-harvest interval for grass hay. A 3 day pre-slaughter interval is retained for livestock.
• Signal word: Labels carry either DANGER or CAUTION, depending on the formulation. Labels must include best management practices for spray drift.

What Is the EU Status of Triclopyr?

The EU status of triclopyr is a renewal review that the EFSA peer review concluded in 2024. The European Food Safety Authority is the EU agency that peer-reviews pesticide risk assessments. Its triclopyr peer review was published in the EFSA Journal on 12 August 2024.

Regulation (EU) No 686/2012 is the EU list of active substances whose approvals are up for renewal. Triclopyr is one of the substances listed in it. The review could not finalise the risk assessment for aquatic organisms. Reliable toxicity data for algae and aquatic plants were not available. Triclopyr and its variants do not meet the criteria for endocrine disruption for humans and non-target organisms.

How Much Triclopyr Is Allowed in Food?

Not much triclopyr is allowed in food: food and water add 49 percent of the reference dose for children. That figure applies to children aged 1 to 6 and describes chronic aggregate risk. Food alone adds less than 1 percent of the reference dose for the general population. For children under one, food adds 3 percent.

Chronic aggregate risk including drinking water accounts for 16 percent of the reference dose for females 13 and older. Updated EFSA chronic consumer exposure to triclopyr reaches 44 percent of the acceptable daily intake. This applies to toddlers in the Netherlands. EFSA estimated intake with revision 3.1 of the PRIMo (Pesticide Residue Intake Model). Triclopyr tolerances exist for grass forage and hay, meat, meat byproducts, milk and eggs, and rice.

Chlorpyrifos is an organophosphate insecticide that produces the same TCP metabolite as triclopyr. The EFSA review is leading to higher MRL proposals for triclopyr. The consumer risk assessment from the consumption of drinking water is not finalised.

What Reference Doses Has Triclopyr Been Given?

Triclopyr has been given five reference values by EPA and EU regulators. The key values include:

• EPA chronic reference dose: The dose is 0.05 mg/kg/day, set from a two-generation rat study with a NOEL of 5.0 mg/kg/day. An uncertainty factor of 100 applies, with 10 for interspecies and 10 for intraspecies differences.
• EPA acute endpoint: The endpoint is a maternal and developmental NOEL of 30 mg/kg/day from a rabbit study. It applies to one-day dietary exposure, not lifetime exposure.
• EU acceptable daily intake: The intake is 0.005 mg/kg bw per day, from a 1-year dog study with a NOAEL of 0.5 mg/kg bw per day. It is the lifetime value of the EU.
• Earlier EU acceptable daily intake: The earlier intake was 0.03 mg/kg bw per day, based on a 2-year rat study. It is higher than the current EU value.
• EU acute reference dose: The dose is 0.3 mg/kg bw, based on the rabbit developmental toxicity study. It is expressed for the triclopyr-butotyl form.

The older 1996 ExToxNet listing shows ADI and MCL as not available and an RfD of 0.025 mg/kg/day. No newer source confirms this value, so the reference values differ between sources.

How Can Triclopyr Be Used More Safely?

To use triclopyr more safely, start with aquatic-labeled products, which can be used to the edge of open water. Pennsylvania requires a site-specific permit for triclopyr applications to vegetation emerging from open water. Aquatic-labeled products with the surfactant included suit non-crop and forestry sites.

Mix glyphosate with triclopyr at the recommended rates for invasive plant control. Glyphosate is a non-selective herbicide often mixed with triclopyr. Use 3 pounds per acre of glyphosate and 2 pounds per acre of triclopyr, on an acid equivalent basis. Adjust the product rate by label, because both herbicides come in different formulations with different concentrations.

What Re-Entry Intervals and PPE Apply to Triclopyr?

The Worker Protection Standard sets re-entry intervals and early-entry equipment for agricultural pesticide use. It is the US farmworker rule for these uses. Re-entry and PPE rules for triclopyr include:

• TEA re-entry interval: The interval is 48 hours for agricultural uses of triclopyr TEA. This is the longer wait of the two forms.
• BEE re-entry interval: The interval is 12 hours for triclopyr BEE. This is shorter than the TEA interval.
• Early-entry protective equipment: Early entry requires coveralls, chemical resistant gloves, protective eyewear, and shoes plus socks for TEA formulations. Both TEA and BEE caused dermal sensitization in test animals.
• Dermal absorption: Calculated dermal absorption is less than 2 percent, based on a study with human volunteers. This is the evidence for low skin absorption.
• Homeowner re-entry: Homeowner re-entry is restricted until sprays have dried and dusts have settled. No hour count is given for homes.