Aminopyralid Residues in Compost

Author: Sam Angima, Oregon State University Small Farms Program and Andy Hulting, Oregon State University Weed Specialist

Publish Date: Winter 2011

Several farmers and gardeners in Washington State lost most of their vegetable crops in 2010 due to the effects of aminopyralid herbicide residues that originated from composted dairy manure. For more information see Herbicide Carryover in Hay, Manure, Compost, and Grass Clipping at

Aminopyralid (marketed as MilestoneTM, ChaparralTM, and OpensightTM ), is an auxinic growth regulator herbicide and will cause damage to sensitive broadleaf plants such as tomato, lettuce, beans and peas. Impacted plants will usually not die when exposed to low residue amounts in compost, but will produce few quality fruits or no fruits.

Aminopyralid breaks down slowly or not at all in the digestive system of a cow or in the composting process, but instead, remains with the organic matter throughout the composting cycle. Problems arise when this compost is used to amend garden soils. Concentrations as low as 1 part per billion (ppb) of aminopyralid will affect sensitive plants such as vegetables. Damage to broadleaf plants includes cupped leaves, twisted stems, distorted apical growing points, and reduced fruit set. See images of damaged plants at:

Conduct Your Own Bioassay

End users like vegetable farmers and gardeners can test for aminopyralid contamination in compost or manure by conducting a bioassay before planting broadleaf plants in soil that has been amended by suspect compost or manure. Sow seeds of plants with known susceptibility, such as peas, beans or tomatoes, in small pots with a mix of the compost material and peat-based potting mix. Place the pots indoors in a warm welllighted place, especially during winter. Find detailed instructions on conducting a bioassay in this WSU document:

Aminopyralid is slowly broken down by microorganisms commonly found in soil.

To dispose of contaminated compost or manure spread them on soils in areas where broadleaf plants will not be grown and incorporate lightly into the soil and irrigate heavily in dry areas. A second and third mixing with the soil may speed the decomposition of the material.

Perform an additional bioassay before planting broadleaf plants into these receiving sites for contaminated materials. Farmers with optional acreage, may plant wheat, oats, or any grass-based cover crop. Grass crops are not susceptible to aminopyralid; planting them provides additional time for aminopyralid to break down. The half-life of aminopyralid is about 35 days. It is broken down by soil microorganisms in warm, moist environments by aerobic process.

Crops harvested from fields tainted with aminopyralid residue cannot be sold. Effected plants will show injury symptoms long before setting fruit. Grow grass-based crops or cover crops in such fields to allow time for aminopyralid to break down.

As you plan for the 2011 season, remember to check your manure or compost source and be prepared to take the necessary steps if you suspect aminopyralid contamination. It is important for everyone in agriculture, livestock growers, composters, and crop farmers to be aware of our actions and to seek information on our inputs.

For more images of plants affected by aminopyralid contamination, check the Washington State University Extension website