Killer Compost Update: Herbicide Damage Still a Major Problem

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Curled plant leaves, like the leaves on this eggplant, are a telltale sign of herbicide damage.
Curled plant leaves, like the leaves on this eggplant, are a telltale sign of herbicide damage.
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Potent chemicals are contaminating compost and manure supplies, a problem we've termed
Potent chemicals are contaminating compost and manure supplies, a problem we've termed "killer compost." 

We need to demand that the Environmental Protection Agency ban the potent, persistent herbicides that continue to contaminate commercial compost and manure supplies.

For more than a decade, gardens and farms have been damaged by compost or mulch that was contaminated with persistent herbicides. These potent chemicals are applied to lawns, pastures, hayfields and roadsides, and continue to be highly toxic even after residues on grass or hay have been composted. When livestock graze on treated pasture or hay, these herbicides even remain potent in their composted manure. We’ve termed this recurring problem “killer compost.” (For one reader’s report on this herbicide contamination, see Dear MOTHER February/March 2013.)

In June 2012, employees at Green Mountain Compost in Williston, Vt., began fielding reports from gardeners about suspected herbicide damage following application of compost purchased at the facility. Initial tests of the compost revealed the presence of two herbicides?–?picloram and clopyralid?–?known to be persistent in compost. Green Mountain, which is now under the management of the Chittenden Solid Waste District (CSWD), immediately suspended sale of its bagged and bulk compost and began seeking the source of the contamination. It also started making reparation arrangements with customers who had reported damage. Picloram and clopyralid are produced by Dow AgroSciences to control broadleaf weeds on turf grass, pastures and rangelands. Both compounds are under restricted use in Vermont; picloram may only be applied by licensed applicators. According to the Vermont Agency of Agriculture, there was no reported use of picloram in the state on any potential composting feedstock between 2009 and 2011.

The Plot Thickens

So how did these chemicals turn up in the compost? In early August 2012, CSWD identified area horse farms as the source of the contamination and sent letters to those farms indicating it could no longer compost their manure. Puzzled as to how the herbicides had gotten into the manure, CSWD asked those farms to indicate which commercial horse feed they had used, and then took the bold step of having samples of that feed tested. Bingo. According to initial lab results, several samples of off-the-shelf Purina horse feed were contaminated with clopyralid at levels between 142 and 465 parts per billion! Susceptible garden plants?–?such as beans, eggplant, peas, peppers, potatoes, sunflowers and tomatoes?–?are harmed at exposure levels as low as 30 ppb, five to 15 times lower than the levels detected in the horse feed.

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