- What are good sources of information on persistent herbicides?
- How can I tell if my compost is contaminated?
- What are persistent herbicides?
- What chemical compounds are in persistent herbicides?
- What plants are sensitive to persistent herbicides?
- What do plants damaged with these herbicides look like?
- How long do persistent herbicides last in soil?
- Where do persistent herbicides come from and how did they get into compost?
- Why don’t persistent herbicides break down in the compost process?
- Does exposure to persistent herbicides in compost pose a health threat?
- Why aren’t labeling requirements effective?
- Why is federal action needed to help prevent herbicide-contamination of compost?
- BioCycle Magazine (Just issued: Persistent Herbicides in Compost)
- Green Mountain Compost
- Washington State University
- Virginia Tech Factsheet: Pyridine Herbicide Carryover: Causes and Precautions
- Ohio State University
- NC State Extension Herbicide Carryover factsheet
How can I tell if my compost is contaminated?
Testing for specific herbicides is expensive and difficult. Before you do that you should use a “bioassay” to test for the general presence of persistent herbicides. This test is one you can do on your own. Find clear instructions on this webpage from the Washington State University Extension: http://whatcom.wsu.edu/ag/aminopyralid/bioassay.html, or download them HERE
If your bioassay indicates the presence of persistent herbicides, then you should have the test redone by a professional lab. Two labs that offer this service are Control Laboratories and Woods End Laboratories.
What are persistent herbicides?
The Environmental Protection Agency (EPA) defines herbicides as chemicals used to manipulate or control undesirable vegetation. Persistent herbicides are a class of systemic herbicides that are used to control a wide variety of broadleaf weeds. These herbicides are formulated to survive multiple years of exposure in a growing environment. We are specifically concerned with the relatively new class of herbicides called “pyridine-carboxylic acids”. They are typically designed for use in hayfields, horse pastures, golf courses, right-of-ways, and lawns to kill off unwanted weeds and to remain effective for several months to years. These herbicides do not impact grasses.
There are a number of compounds that fall into the category of persistent herbicides. The most prevalent are Clopyralid (Dow Agrosciences), Aminopyralid (Dow Agrosciences, 2005), Aminocyclopyrachlor (DuPont, 2010), and Picloram (Dow Agrosciences). Less prevalent compounds in the same class include fluroxypyr, dopyralid, and triclopyr. Many of these compounds appear on labels in slightly different variations making identification by the untrained applicator or a testing lab difficult.
What plants are sensitive to these herbicides?
Plant families sensitive to clopyralid include
• Legumes, such as peas, beans, lentils, and clover
• Solanaceous, such as tomatoes and potatoes
• Composite, such as sunflower, petunias, daisies, and asters
• Other plants, such as carrots, carnations, lupines, and lettuce.
What do plants damaged with these herbicides look like?
Damaged plants will show:
• Stunted growth: the main growth tip stops growing and the lateral buds begin to grow
• Reduced fruit set
• Cupping of leaves
• Failure of secondary leaves to grow after the seed leaves emerge
• In legumes, compound leaves stay single
For photos of herbicide damage:
http://puyallup.wsu.edu/soilmgmt/Clopyralid.html (at bottom of page)
How long do persistent herbicides last in soil?
Depending on the type of herbicide and the level of concentration in the soil, persistent herbicides can last anywhere from several months to three or more years before completely breaking down into inert compounds. The length of time depends upon a variety of factors, including the type and moisture content of the soil.
Why don’t persistent herbicides break down in the compost process?
Commercial composting involves a process of intense and prolonged biological activity at high temperatures. This environment not only results in rapid degradation of food scraps and other feedstocks, but is also extremely effective at degrading the vast majority of any potential herbicide and pesticide residues into their harmless constituent pieces. Persistent herbicides are relatively new compounds that have been formulated by the manufacturers specifically to be resistant to biological degradation. While most residual traces of herbicides typically breakdown in a compost pile in a matter of days, the molecular bonds joining these particular compounds can be resistant for months or even years.
Does exposure to clopyralid in compost pose a health threat?
No. According to the EPA, it is not harmful to people or animals at the low levels present in compost. In fact, people that have applied this herbicide to their lawns have a much higher concentration of clopyralid in their soil than will be found in compost. The primary environmental concern from this herbicide is its effect on sensitive plants. If vegetables grown in soil treated with herbicide-contaminated compost manage to produce anything, the vegetables are safe to eat.
Where do persistent herbicides come from and how did they get into compost?
The most common pathway known for persistent herbicides making their way into compost is through manures and bedding as well as leaf and yard debris. Depending on the region, these compounds are used in variable amounts on horse pastures, hay and grain fields, golf courses, right-of-ways, and lawns. The resulting hay, grass, or digested and excreted materials are required to be disposed of somewhere other than a garden or compost facility, or introduced back onto the land of original application. The labeling requirements for many of the persistent herbicides state that manures from animals grazing in treated areas or hay and grass clippings from treated areas are not to be sent to a compost facility.
- The information on the use in a field does not get communicated to the hay buyer
- The hay buyer may sell the hay to a dairy or horse owner without communicating the use of the herbicide-based restrictions
- The farmer does not communicate the information to the hauler, who brings the manure to a composter
• In urban/suburban use:
- The applicator may not tell the home or business owner
- The home or business owner sets the grass at the curb for collection
- The test is very expensive, over $300 per test
- Incoming materials are too heterogeneous to be effectively sampled
- You have to know exactly which version of which chemical you are testing for. If you test for clopyralid, you may miss aminocyclopyrachlor.
- It is unfair to place this financial burden on the composter. At the very least, the chemical companies need to establish and make available a fund that composters can use for testing compost feedstocks and products, and a compensation plan for positive contamination.
What federal action is needed to help prevent herbicide-contamination of compost?
Some states have prevented or curtailed use, but products often move across state borders, so action is needed at the federal level. Despite the fact that the labels are ineffective, the chemical manufacturers are protected by the labeling restrictions, and are thus shielded from being held liable. As we increase recycling and composting – widely recognized as vital to sustainability – these types of persistent herbicides will become more problematic. Requiring ALL herbicides to meet a compostability standard would prevent the problem in the future and spur market introduction of better, more environmentally responsible products. The EPA must initiate a Special Review Process for this entire class of chemicals, which would allow it to assess the situation and take appropriate action. A moratorium on continued sale and use is necessary until the Review Process is complete. Herbicide manufacturers need to be held financially responsible for lab tests and mitigation costs to offset the financial burden on compost producers.