Registration Review of Pyridine and Pyrimidine Herbicides

Registration Review of Pyridine and Pyrimidine Herbicides

• Basic information on use
• Human health
• Ecological health
• Compost education and mitigation
• List of pyridines/pyrimidines

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Basic Information on Uses

The pyridine and pyrimidine classes are herbicides with selective toxicity to broadleaf weeds. These herbicides are used to control weeds in both agricultural and non-agricultural settings. Use sites vary from herbicide to herbicide. Example use sites are listed below.

Herbicide	Summary of Use Sites
Aminocyclopyrachlor	Privately-owned and non-hayed rangeland, grassland managed as rangeland, rights of ways, roadsides, industrial areas.
Aminopyralid	Wheat, corn, pasture, rangeland, conservation reserve program land, forests, rights of ways, fallow, non-agricultural uncultivated areas, industrial areas.
Clopyralid	Alfalfa, apples, asparagus, barley, cabbage, canola, cherries, corn, dry beans, fallow, pasture, peaches, pecans, spinach, strawberry, sugar beets, corn, wheat, turf (including athletic fields, golf courses, parks, industrial areas), ornamental trees, rights of ways.
Dithiopyr*	Turf (including residential lawns, sod farms, golf courses, athletic fields, industrial areas), ornamental plants, rights of ways, and non-crop areas.
Fluroxypyr	Barley, corn, grasses grown for seed, forage, pasture, oats, onion, pome fruits, rice, rye, sorghum, triticale, wheat, fallow, conservation reserve program land, industrial areas, rights of ways, turf (including residential lawns, recreational areas, ornamental lawns).
Picloram	Barley, oats, wheat, pasture, fallow, forests, conservation reserve program land, rights of way, industrial areas, fencerows/hedgerows.
Triclopyr	Pasture, rangeland, forests, turf (including golf course, residential areas, recreational areas, and industrial areas), rights of ways, fencerows.

^*Although dithiopyr is a pyridine herbicide, it has a different mode of action than the herbicides in the rest of the group.

Human Health

There are no human health risks of concern for any registered uses of aminopyralid, clopyralid, dithiopyr, fluroxypyr, picloram and triclopyr. EPA has not yet completed the human health risk assessment for aminocyclopyrachlor in registration review.

Ecological Health

Ecological risks are primarily for non-target terrestrial plants through spray drift and runoff. Certain herbicides in the pyridine/pyrimidine class can persist in treated plant materials, and when treated materials are recycled into compost, can cause harm to plants in gardens and ornamental plots where the compost is applied. Pathways into compost include residues in treated turf clippings, hay from treated pastures, straw from treated wheat and other cereal grains, and manure from animals that have grazed in treated areas. 

Compost Restrictions

EPA is working to address compost contamination concerns for certain persistent herbicides during registration review.

Clopyralid, aminopyralid, and picloram are more persistent compared to the other herbicides in this group. Through the interim decisions for aminopyralid, clopyralid, and picloram, the following mitigation measures are required to address potential residues in compost: 

• Prohibition of off-site use of treated plant materials and manure from grazing animals for compost and animal bedding/feed until 18 months after application to allow for residues to decline.
• Requirement that livestock be grazed on forage that haven’t been treated for three days before moving to a site where manure is collected, or sensitive crops are grown. 
• Removal of use on residential turf from all labels (clopyralid only).
• Notification by applicators to landowners/operators of compost restrictions for pasture (all three chemicals) and turf sites (clopyralid only). Applicators must keep records of the notification for two years.
• Updating compost pictogram on pesticide labels showing growers/operators how to manage treated materials. 
• Requirement for registrants to participate in a stewardship program and provide educational outreach for applicators, growers, land managers/operators, and others affected by herbicide residues in compost. 

For triclopyr and fluroxypyr (the less persistent herbicides), the following measures are required:

• Prohibition of the use of treated plant materials or manure from animals that have grazed or consumed forage from treated areas for composting until 30 days after application.
• Requirement that livestock be grazed on forage that haven’t been treated with the pesticides for three days before moving to a site where manure is collected, or sensitive crops are grown.

EPA is preparing additional educational resources for land managers and others affected by herbicide residues in compost.

List of Pyridines/Pyrimidines

Chemical	Docket Numbers	Status
Aminocyclopyrachlor (ACP)	EPA-HQ-OPP-2020-0384	Preliminary Work Plan Final Work Plan
Aminopyralid	EPA-HQ-OPP-2013-0749	Interim Decision Proposed Interim Decision Draft human health risk assessment Draft ecological risk assessment
Clopyralid	EPA-HQ-OPP-2014-0167	Interim Decision Proposed Interim Decision Draft human health risk assessment Draft ecological risk assessment
Dithiopyr*	EPA-HQ-OPP-2013-0750	Interim Decision Proposed Interim Decision Revised draft human health risk assessment Revised draft ecological risk assessment
Fluroxypyr	EPA-HQ-OPP-2014-0570	Interim Decision Proposed Interim Decision Draft human health risk assessment Draft ecological risk assessment
Picloram	EPA-HQ-OPP-2013-0740	Interim Decision Proposed Interim Decision Draft human health risk assessment Draft ecological risk assessment
Triclopyr	EPA-HQ-OPP-2014-0576	Interim Decision Proposed Interim Decision Draft human health risk assessment Draft ecological risk assessment

^*Although dithiopyr is a pyridine herbicide, it has a different mode of action than the herbicides in the rest of the group.