Clopyralid, the herbicide that doesn’t break down in compost or manure, is suspected in several Midwestern cases of plant damage this summer. According to the Environmental Protection Agency, clopyralid use is not limited to Washington state, where it has been restricted because of widespread contamination of compost. It is also used in Oregon, Idaho, Montana, Wyoming, Colorado, Kansas, Nebraska, South Dakota, North Dakota, Minnesota and Michigan. If you farm in those states, you should be extremely cautious about bringing compost, hay, straw or animal manure into your fields until you are sure that it is not contaminated.
Laurie Hodges, a University of Nebraska Extension specialist, wrote in a recent newsletter to Nebraska farmers: “If a horse consumes treated forage, there is sufficient clopyralid in the horse urine to affect non-target broadleaf plants. The label has a “cleansing” period of 7 days on untreated pasture prior to moving livestock to land used for growing susceptible broadleaf crops.
“This is why you need to know what was done to the field providing the forage, hay, or straw used by the animals producing the manure – or composted manure – you are getting to improve the soil or prepare potting mixes for your horticultural crops. The compound is biologically active at very low concentrations.
“Various vegetable rotation intervals are listed on the label from 10.5 months through 18 months.. Sweet corn, strawberries, asparagus, onions, and cole crops are among the more tolerant crops.”
Clopyralid is made by Dow AgroSciences and sold under the trade name Stinger. Low levels of contamination can cause plant damage, which usually exhibits as twisted stems and cupped leaves.