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The information in this profile may be out-of-date. It was last revised in 1996. EXTOXNET no longer updates this information, but it may be useful as a reference or resource.

Please visit the National Pesticide Information Center (NPIC) to find updated pesticide fact sheets. If you don't find a fact sheet related to your question, feel free to call 1-800-858-7378. NPIC is open five days a week from 8:00am to 12:00pm Pacific Time.

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E X T O X N E T

Extension Toxicology Network

Pesticide Information Profiles

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Oregon State University, the University of Idaho, and the University of California at Davis and the Institute for Environmental Toxicology, Michigan State University. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.

EXTOXNET primary files maintained and archived at Oregon State University

Revised June 1996

Picloram

Trade and Other Names: Commercial products containing the compound include Access, Grazon, Pathway, and Tordon. It may be used in formulations with other herbicides such as bromoxynil, diuron, 2,4-D, MCPA, triclorpyr, and atrazine. It is also compatible with fertilizers.

Regulatory Status: Picloram is a slightly toxic compound in EPA toxicity class III. Products containing it must bear the Signal Word CAUTION on the label. All products except for Tordon RTU and Pathway are Restricted Use Pesticides (RUPs). RUPs may be purchased and used only by certified applicators.

Chemical Class: pyridine compound

Introduction: Picloram, in the pyridine family of compounds, is a systemic herbicide used for control of woody plants and a wide range of broad-leaved weeds. Most grasses are resistant to picloram, so it is used in range management programs. Picloram is formulated either as an acid (technical product), a potassium or triisopropanolamine salt, or an isooctyl ester, and is available as either soluble concentrates, pellets, or granular formulations. The materials in this document refer to the technical acid form unless otherwise indicated.

Formulation: Picloram is formulated either as an acid (technical product), a potassium or triisopropanolamine salt, or an isooctyl ester, and is available as either soluble concentrates, pellets, or granular formulations.

Toxicological Effects:

• Acute toxicity: Picloram is slightly to practically nontoxic via ingestion, with reported oral LD50 values of greater than 5000 mg/kg to 8200 mg/kg in rats, 2000 to 4000 mg/kg in mice, and approximately 2000 mg/kg in rabbits [1,58]. The reported dermal LD50 in rabbits is greater than 4000 mg/kg, a level which produced no mortality or toxic signs [58,6]. This indicates slight toxicity via the dermal route as well. Technical picloram is reported to cause no skin and moderate eye irritation in the rabbit, and to cause no skin sensitization in the guinea pig [1,58]. Some formulations have caused mild or slight skin irritation and skin sensitization in test animals [58]. The technical grade is moderately toxic by inhalation, with a reported 4-hour inhalation LC50 of greater than 0.35 mg/L [3]. Formulated products may show a lesser toxicity via this route [58]. There is no documented history of human intoxication by picloram, so symptoms of acute exposure are difficult to characterize.
• Chronic toxicity: Male mice receiving picloram at dietary doses of 1000 to 2000 mg/kg/day over 32 days showed no clinical signs of toxicity nor changes in blood chemistry, but females did show decreased body weight and increased liver weights [6,8]. Liver effects were also seen in rats at very high doses of 3000 mg/kg/day over an exposure period of 90 days, and above 225 mg/kg/day for 90 days [58]. Dogs, sheep, and beef cattle fed low levels of picloram for a month experienced no toxic effects. The ester and triisopropanolamine salt showed low toxicity in animal tests [58]. Picloram may show additive effects if mixed with other herbicides such as 2,4-D [118].
• Reproductive effects: In multi-generational studies, pregnant rats exposed during critical periods of gestation to doses of about 180 mg/kg/day of picloram showed no changes in fertility [58]. The fertility of pregnant mice fed 15 mg/kg/day for 4 days before and 14 days after mating was not adversely affected [8]. Other studies showed no effects on fertility or fecundity in rats at doses as high as 1000 mg/kg/day [58]. Picloram does not appear to cause reproductive toxicity.
• Teratogenic effects: No teratogenic effects were seen in the offspring of pregnant rats exposed during gestation to 400 mg/kg/day of the acid or potassium salt, or to 1000 mg/kg/day of the ester or other salt [58]. At 2000 mg/kg/day, maternal toxicity was noted but did not induce malformation in the pups [8]. It appears that picloram is not teratogenic.
• Mutagenic effects: One test has shown that picloram is mutagenic (to the bacterium Saccharomyces cerevisiae) and another test has shown that it is not mutagenic (Ames test) [118]. In tests for unscheduled DNA synthesis and structural chromosome aberrations, the results were also negative [58]. These data suggest that picloram is either nonmutagenic or weakly mutagenic.
• Carcinogenic effects: Mice fed average doses of 18 mg/kg/day or 30 mg/kg/day for 80 weeks and observed for another 10 weeks did not display any carcinogenic effects [8,118]. Male rats fed 17.5 or about 40 mg/kg/day for 80 weeks and observed for 33 weeks showed no carcinogenicity, but females developed benign liver tumor nodules [58]. Other tests have indicated an increased incidence of cancer among animals treated with picloram, but these data are difficult to interpret due to possible interference of hexachlorobenzene contaminants [8,118]. These data suggest that picloram is noncarcinogenic or weakly carcinogenic.
• Organ toxicity: Animal studies show the target organs for picloram to be the liver and kidneys.
• Fate in humans and animals: Picoloram was rapidly absorbed through the gastrointestinal tract in studies using human volunteers, and was excreted unchanged in the urine [119]. Half of the product was excreted within a day or so. Skin absorption is minimal [119]. Rats showed similar results, with administered doses excreted virtually unchanged in urine and feces within 48 hours [119]. Picloram does not accumulate in fat [119]. No measurable residues were found in milk from cows fed small amounts of the herbicide in their diets [8]. At higher levels of exposure, milk levels of picloram were low (0.05 to 0.29 ppm) and declined rapidly upon withdrawal of picloram from the diet [8].

Ecological Effects:

• Effects on birds: Picloram is slightly to practically nontoxic to birds; the acute oral LD50 is greater than 2000 to 5000 mg/kg in ducks, pheasants and quail, with no mortality seen at even the highest levels [6].
• Effects on aquatic organisms: Picloram is slightly to moderately toxic to fish and aquatic invertebrates. The reported 96-hour LC50 values for picloram are 19.3 mg/L in rainbow trout, 14.5 mg/L in bluegill sunfish, and 55 mg/L in fathead minnow [58]. The 48-hour LC50 in Daphnia is 50 mg/L, indicating moderate toxicity [58]. Most salts are of similar or lesser toxicity, but the isooctyl ester may be highly toxic. The reported 96-hour LC50 for the isooctyl ester in rainbow trout is 4 mg/L, and in channel catfish is 1.4 mg/L [118]. Other LC50 values in aquatic invertebrates ranged from 10 to 68 mg/L [8]. Picloram is not expected to accumulate appreciably in aquatic organisms; the measured bioconcentration factor in bluegill sunfish was less than 0.54 [9].
• Effects on other organisms: The compound is nontoxic to bees [1].

Environmental Fate:

• Breakdown in soil and groundwater: Picloram is moderately to highly persistent in the soil environment, with reported field half-lives from 20 to 300 days and an estimated average of 90 days [11]. Photodegradation is significant only on the soil surface and volatilization is practically nil [58]. Degradation by microorganisms is mainly aerobic, and dependent upon application rates [58]. Increasing soil organic matter increases the sorption of picloram and increases the soil residence time [58]. Picloram is poorly bound to soils, although it is bound better by soils with higher proportions of soil organic matter [11]. It is soluble in water, and therefore may be mobile [1]. These properties, combined with its persistence, mean it may pose a risk of groundwater contamination. Picloram has been detected in the groundwater of eleven states at concentrations ranging from 0.01 ug/L to 49 ug/L [9].
• Breakdown in water: In laboratory studies, sunlight readily broke down picloram in water, with a half-life of 2.6 days [9,58]. Herbicide levels in farm ponds were 1 mg/L directly following spraying, and decreased to 0.01 mg/L within 100 days, primarily due to dilution and the action of sunlight [6].
• Breakdown in vegetation: Picloram is readily absorbed by plant roots, less so by the foliage, and is readily translocated throughout plants. It remains stable and intact in plants [58].

Physical Properties:

• Appearance: Picloram is a colorless crystal [1].
• Chemical Name: 4-amino-3,5,6-trichloropyridine-2-carboxylic acid [1]
• CAS Number: 1918-02-1
• Molecular Weight: 241.48
• Water Solubility: 430 mg/L @ 25 C [1]
• Solubility in Other Solvents: v.s. in acetone, ethanol, benzene, and dichloromethane [1]
• Melting Point: Decomposes @ 215 C [1]
• Vapor Pressure: 0.082 mPa @ 35 C [1]
• Partition Coefficient: 0.1461 [58]
• Adsorption Coefficient: 16 [11]

Exposure Guidelines:

• ADI: Not Available
• MCL: Not Available
• RfD: 0.07 mg/kg/day [13]
• PEL: 5 mg/m3 (8-hour) (respirable fraction ) [14]
• HA: 0.5 mg/L (lifetime) [120]
• TLV: Not Available

Basic Manufacturer:

DowElanco
9330 Zionsville Road
Indianapolis, IN 46268-1054

• Phone: 317-337-7344
• Emergency: 800-258-3033
  

References:

References for the information in this PIP can be found in Reference List Number 10

DISCLAIMER: The information in this profile does not in any way replace or supersede the information on the pesticide product labeling or other regulatory requirements. Please refer to the pesticide product labeling.