One Year Later · Persistent Herbicides in Compost

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ONE YEAR has passed since the Washington State University (WSU) composting facility and the Spokane Regional Compost Facility discovered traces of persistent herbicides in their composts. In Spokane, the source of contamination is a compound called clopyralid. Compost contamination at WSU initially involved the herbicide picloram, but clopyralid has since been detected in the compost also. In both cases, the compost damaged sensitive plants at local gardens and nurseries. Several articles published at that time brought the issues to the forefront (see “Dealing with Herbicide Residues in Compost,” September, 2000 BioCycle and also the references, Bezdicek, et al. 2000, and Miltner and Stahnke, 2000). Clopyralid contamination has since been reported at a yard trimmings facility in Cheney, Washington and at a horse facility in Whitman County Washington. Incidents in Pennsylvania and New Zealand have also come to light.

Today, both the WSU and Spokane facilities are still dealing with the problem of persistent herbicides. Facility managers, regulators, public agencies, researchers and the herbicide manufacturer continue to seek solutions to the problem, ranging from on-site treatment to regional product bans.


Clopyralid is an active ingredient in several herbicide products that have a wide variety of applications (Table 1). The most prominent product, and the one implicated in the Spokane situation, is Confront, made by Dow AgroSciences (Dow). Confront is used on lawns to control dandelions, clover, and other broadleaf plants. It was introduced in 1989 to replace existing herbicides that required multiple applications — from three to five applications per season to one or two. Clopyralid also is registered in a number of formulations used on cereals, grass hay, sugar beets, mint, asparagus, strawberry, blueberry and Christmas trees. At WSU, the source of clopyralid contamination is believed to be from grass hay and straw, purchased for its livestock operations on and off campus. Thus clopyralid is a potential concern for composting facilities taking yard trimmings or agricultural feedstocks.

Picloram is used to control weeds for agricultural and industrial applications including pasture, rangeland, railroads and power lines. It is the less worrisome of the two herbicides since treated residuals are less likely to be collected for composting.

Clopyralid and picloram are classified as pyridine carboxylic acid herbicides (see Figure 1). They are growth-regulator type herbicides. Like 2,4-D and dicamba, they work by mimicking plant growth hormones called auxins. The effect of these chemicals is to cause the plant to grow abnormally. They are quite water soluble and mobile in soil. One advantage of these chemicals is their low toxicity for animals, including humans.

Typical symptoms of clopyralid phytotoxicity are shown in Figure 2. Although plants may appear normal, the loss of “apical dominance” will likely prevent fruit set. Some leaves that are supposed to be compound become single leaves. Side shoots may develop where they should not be. In legumes, cupping is a typical symptom and trifoliate leaves fail to develop. The most susceptible crops can be sensitive to clopyralid at the ppb level. Clopyralid is extremely toxic to sunflower, legume crops and solanaceous plants, including tomatoes and potatoes. Effects on these plants can be seen at levels of 10 parts per billion (ppb) or less. According to Dow’s literature, this is roughly 100 times lower than the tolerance allowed on asparagus, 50,000 times lower than the tolerance allowed on grasses, and 300 times lower than allowed on barley grain. Concentrations above these low levels are seldom seen in the real world, but they point out the potential for contaminated feedstocks to affect sensitive crops.

Because of the relatively high tolerance allowed for clopyralid for public safety, health, wildlife and environmental concerns, most laboratories do not routinely measure either picloram or clopyralid below 50 ppb. Consequently, potentially-injurious levels in straw, hay, compost, and other residuals are often not detected. At WSU, an initial series of analytical tests of the compost did not reveal any contamination from any suspected herbicides. However, after consultation with a local commercial analytical laboratory, procedures were modified and detection levels increased in sensitivity from about 50 to about 1 ppb.

Most herbicides break down rapidly after application. In two review articles on pesticides in composting (Buyuksonmez et al. 1999; 2000), herbicides were generally considered to break down during normal composting. However, some of those in the pyridine carboxylic acid group such as clopyralid break down very slowly, especially during composting. When ingested by animals, these compounds pass through into the urine quickly and without significant degradation. Regulations requiring “weed-free” straw for highway revegetation projects, for local organic gardens, and for “weed free hay” on Federal lands may not take into account the potential for contamination of herbicides used to create the “weed free” product.

The label recommendations for clopyralid products state “Do not use compost containing grass clippings in the growing season of application.” This label recommendation was based on trials at Michigan State University (Vandervoort et al., 1997). Although the research reported clopyralid concentrations decreased during composting, after one year the reported concentrations ranged from 100 to 1,300 ppb of clopyralid in compost made from treated grass clippings. However, these concentrations are 10 to 100 times the level known to cause injury in sensitive plants. Thus, the label is probably not sufficient to protect composting facilities because damaging levels may persist beyond the season of application, as occurred in Spokane and at WSU.


The Spokane Regional Compost Facility at Colbert, owned by the city’s Spokane Regional Solid Waste System, has been operating since 1993. Until recently, it operated an open windrow site, processing 25,000 tons/year. The clopyralid problem first surfaced in 2000 when injury was noted on tomatoes grown in containers with compost from the Colbert facility. The contamination persisted and was traced to lawn clippings brought to transfer stations and later to the Colbert facility for composting. After investigation by the Washington Department of Agriculture, the sale of the compost was halted. The facility now has 25,000 cubic yards (cy) of unsold compost.

Samples of this material taken in January, 2001 (nine months after the problem was discovered) still showed clopyralid residues of 31 to 75 ppb. A limited amount of the compost from the 1999-2000 season has been sold to landscapers and other wholesalers but with the understanding that it could be used only on turf, not gardens (sales always have been wholesale — the facility’s permit doesn’t allow retail sales). Purchasers had to sign a waiver and name the city on their insurance policy. The Spokane Regional Solid Waste System has received only a few claims for damage from the compost, and they have been settled.

In fall of 2000, Norcal of Spokane took over operation of the Colbert facility and is presently producing compost using an Ag-Bag aerated windrow system (previously turned windrows). It was hoped that a change in the composting method would lead to better degradation of the herbicide. However, traces of clopyralid still persist, preventing the sale of the compost to the public. This compost was produced in fall 2000 from feedstocks consisting of leaves, traces of grass clippings, and other yard trimmings. Norcal sampled the bagged material in March 2001 and clopyralid was present at 57 to 67 ppb. In May 2001, the finished compost from these bags failed a bioassay test. Contamination levels in this material have now been tested at 73 and 80 ppb.

In April 2001, the city and Spokane county asked Dow to voluntarily halt distribution of clopyralid in the Spokane area, at least temporarily, and they have agreed to do so for residential uses. Application will still continue at golf courses where grass cycling is practiced. Grass from golf courses and parks is not taken to the composting facility. Dow has prepared a flyer for landscape companies asking their cooperation and suggesting alternatives they can use. Norcal continues to accept all yard trimmings material, including grass. A screening process for raw feedstocks is being considered but has not been implemented. However, the city has increased its public education campaign, distributing information telling people why herbicide-treated grass cannot be composted.

Norcal is working with Dow on a research project seeking a way to break down clopyralid in compost. The Scope of Work includes testing residues in grass from test plots at different intervals after spraying, as well as composting in a laboratory setting under a variety of conditions.


In Pullman, Washington, 80 miles south of Spokane, WSU produces 25,000 cy of compost from animal manure and bedding, collected from the university farms and research facilities. The product is sold to nurseries in eastern Washington and northern Idaho as straight compost and is also blended by nurseries in a 50-50 compost/soil mix.

WSU first faced the herbicide issue in 2000 when picloram, a compound closely related to clopyralid, was detected in its composts. The source of contamination was eventually traced to a pasture sprayed with Tordon-101, a product containing picloram and 2,4-D — both herbicides used to control broadleaf weeds. This pasture was later harvested for hay (off label), which was fed to livestock. The manure and bedding taken to the WSU composting facility carried the picloram with it. Compost was sold to nurseries and later to homeowners who discovered plant-deforming symptoms on peas, beans, tomatoes, potatoes, sunflower and certain shrubs. The incident put WSU on the alert, and they instituted a regular testing program for herbicides, including both analytical tests and bioassay with plants.

While the picloram contamination resulted from an isolated incident, in the spring of 2001, compost from WSU’s windrow system was found to contain clopyralid in concentrations from traces to over 200 ppb. The source of contamination is believed to be from grass hay and straw used in their animal feeding operations. During 2000 and 2001, WSU performed extensive testing on hay fed to livestock and straw for bedding to identify sources of contamination. Now WSU is certifying vendors of grass hay and straw. Dan Caldwell, farm and compost manager at WSU, has told vendors that they have to be certified by WSU to sell grass hay and straw. To be certified, vendors will have to guarantee that their product is free from herbicide contamination. “Some may decide not to participate. Others will become certified vendors, and they are the only ones we will purchase from,” Caldwell says.

Since the original incident, WSU has since taken a proactive position, advising homeowners on the contamination and offering advice and compensation to affected compost users. WSU initiated community meetings with the public and began a bioassay test program for soils suspected of being contaminated from the compost. A bioassay involves growing sensitive plants for a few days and watching for signs of stress. WSU used tomatoes, beans, and peas as indicator plants. Analytical tests were conducted to confirm the presence of picloram and clopyralid. Remediation strategies such as frequent watering and cover crops were recommended to facilitate breakdown of the chemicals.

This spring, WSU has tested garden soil from the public for possible herbicide carryover from last year’s application. When needed, WSU has provided activated charcoal to homeowners as a means of reducing or eliminating the damage and allowing gardeners to plant their gardens this year. In some cases, where the recommended amounts of compost were applied (one inch in depth or less), the soils were sufficiently free of the herbicide to grow a garden. Where higher amounts of compost were applied in 2000, sensitive plants were still affected in 2001. WSU has not sold any compost for the past two years.


In today’s world, where yard trimmings are banned from many landfills, composting is one of the only means for responsible stewardship. The recycling and composting industries are worried that persistent herbicides, such as clopyralid, might jeopardize the significant gains in public confidence regarding organics recycling and compost markets. There is also a financial element. Millions of dollars have been invested in many composting facilities for the public and commercial companies to bring yard trimmings for composting, and to purchase compost. At the Spokane Regional Compost facility, 25,000 cy from 2000 remain unsold and clopyralid contamination in 2001 has prevented sale of compost to the general public. At WSU, the cost from two years’ loss of revenue, claims, bioassay and analytical testing, and additional labor has totaled about $250,000.

The Washington Organics Recycling Council (WORC) is asking all composting facilities to start performing bioassay growing trials to monitor for clopyralid. These tests are much cheaper than chemical analysis. “If they do find herbicide effects in their bioassays, we recommend they follow up with chemical tests,” said Jeff Gage, WORC president and a member of the board of directors of the U.S. Composting Council. Gage said Washington has about 30 licensed public and private composting facilities.

What should be done to protect the recycling and composting communities from inadvertent contamination by persistent herbicides, and whose responsibility is it? Voluntary removal of products locally, as in Spokane, is one option. Restricting the sale on a county or state basis is another. A third option is controlling the feedstocks that enter a composting facility. However, this can be difficult as there is often a disconnect between the applicator of the lawn care services and the service of picking up lawn clippings and distributing them to appropriate treatment sites. With turf applications, the ultimate solution is to grass cycle, a wise option in any case.

One fact is apparent — relying on the herbicide label restrictions is not enough. Herbicides from feedstocks treated with clopyralid can remain in compost for sale to the public, even though the label may have been followed. The public is often not aware that the intent to have weed-free lawns and crops may have hidden dangers from persistent herbicides.

Public utilities in Washington and Oregon have sent a letter to the U.S. Environmental Protection Agency urging them to reconsider its criteria for registration and re-registration of herbicides to include the ultimate end use of urban yard trimmings, food scraps, and agricultural wastes (see “Composting View” in this issue). This letter emphasizes that the fate of residual herbicides must be tested in the normal 60 to 90 day composting cycle to assure that no residual herbicide remains. Some facilities compost for even shorter periods of time. The letter emphasizes that the ultimate responsibility in dealing with clopyralid residues should not be on the compost producers, but on the manufacturer of herbicide-containing products.

Should those registering persistent herbicides give more consideration to the potential end use of residues treated with their products? With composting now being a standard practice of recycling, should not the entire waste reduction and pesticide industries assure that chemicals applied to composting feedstocks be compatible with the potential end uses of the compost? Better awareness and communication between chemical suppliers, lawn care providers, homeowners, other chemical users, and the composting community is needed to recognize the risks of using persistent herbicides. Those who collect, haul and process feedstocks for composting don’t always know the history of the feedstocks.


Regardless of the pesticide registration debate, composting facilities should take note of the possibility that persistent herbicides can endure in compost at damaging levels. So far, clopyralid and picloram are the only herbicides that been implicated, and only at a few facilities. Nevertheless, a few prudent practices might avoid big problems. Facility operators should become familiar with sources of feedstocks and inquire whether persistent herbicides have been used. A bioassay should be conducted on all feedstocks suspected of being treated with clopyralid, especially grass clippings, as well as the finished compost. If symptoms of herbicide damage show, bioassays should be followed up with analytical tests. It is important to check for salinity and maturity to verify that the symptoms reflect only herbicide damage.

As for the big picture, regulatory agencies need to reexamine the criteria used in considering the registration and reregistration of persistent herbicides to include all possible end uses of plant residuals. Herbicide manufacturers must recognize that the persistence of herbicides is a critical concern to all end uses of plant residuals, including recycling and composting. The composting industry urges herbicide manufacturers to reexamine the chemistry and formulation of existing and new herbicides for persistence in the environment so that recycling and composting are not at risk.

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