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Registration Review of Pyridine and Pyrimidine Herbicides

Registration Review of Pyridine and Pyrimidine Herbicides

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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.

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Human Health

There are no human health risks of concern for this class of herbicides.

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, and manure from animals that have grazed in treated areas.

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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 decision for clopyralid, EPA required the following mitigation measures. For aminopyralid and picloram, EPA has proposed the following mitigation measures via the proposed interim decision for these pesticides.

  • Prohibit the transport of treated plant matter and manure from animals that recently grazed in treated areas for offsite composting for a period of time until residues have adequately declined.
  • A clean-out period of at least 3 days for animals fed with treated plant materials.
  • Removal of use on residential turf language from all labels (clopyralid only).
  • For pasture and turf sites: applicators must notify landowners/operators of compost restrictions. Applicators must keep records of the notification for two years.
  • Updated compost pictogram on pesticide labels showing users when not to compost materials.
  • Registrants to participate in a stewardship program and provide educational outreach for those affected by herbicide residues in compost.

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

  • Prohibit 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.
  • A clean-out period of at least 3 days for animals fed with treated plant materials.

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

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List of Pyridines/Pyrimidines

Chemical Docket Numbers Status

Aminocyclopyrachlor (ACP)

EPA-HQ-OPP-2020-0384

Preliminary Work Plan

Aminopyralid

EPA-HQ-OPP-2013-0749

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

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.

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