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When Compost Kills

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When Compost Kills

How worried should you be about Milestone herbicide?

It’s been a tough year for gardeners. A wet, soppy, cold spring followed by a long period of time with hot sun and no rain has left many—especially those without greenhouses—to wonder whether their tomatoes will make it to red or if corn will have time to grow. Some gardeners, however, saw their tomatoes or beans grow in weirdly malformed shapes, and are questioning whether some other factor might be at work—specifically, whether Milestone™, an ubiquitous herbicide, might have made its way into their garden, perhaps via the compost pile.

“Milestone herbicide warning to All Gardeners,” the email read. “Do not use any manure or urine on your garden from animals feed (sic) grass or foliage treated with Milestone herbicide. It will harm your crop, garden soil, and perhaps your health.”

How big is this threat?

Milestone, (active ingredient aminopyralid and made by Dow AgroSciences) acts to kill broadleaf plants, which includes most of the food plants grown in a garden. Generally used to keep down weeds in grass fields (including hay), the herbicide taken up in the hay and then eaten by farm animals is still present in the animals’ manure up to three days later. Dow AgroSciences warns, on their website and on product labels, that this manure should not be used on gardens, or in areas where the growth of broadleaf plants is encouraged. Hay or straw grown with this herbicide should not be used as mulch on a garden.

So much for the shoulds and should nots.

Milestone first came to public attention in the United Kingdom back in 2008, when thousands of gardeners lost their crops after inadvertently adding this herbicide to the mix via manure in their compost. 

Dow warns, “Aminopyralid is usually decomposed over the growing season by microorganisms in soil. Residues in manure alone break down slowly, but may break down faster if incorporated into soil and rototilled or turned over regularly.” On its UK website, it also states: “Affected manure must not be given or sold on to gardeners or allotment holders as it will affect any sensitive crops that they subsequently grow in soil it has been incorporated into.”

So, if animals eat grasses where aminopyralids were used to keep down weeds, then said animals’ excrement should not be put into your garden compost mix. Seems simple.

Despite the UK experience, aminopyralids began to find their way into U.S. gardens. The North Carolina Cooperative Extension service reports, “Many farmers and home gardeners have reported damage to vegetable and flower crops after applying horse or livestock manure, compost, hay, or grass clippings to the soil,” due to herbicide contamination of manure. Further, they state, “Depending on the situation, the herbicides can be deactivated in as few as 30 days, but some field reports indicate that complete deactivation and breakdown can take several years. Hays have been reported to have residual herbicide activity after three years’ storage in dry, dark barns. Degradation is particularly slow in piles of manure and compost. When mulches, manures, or composts with residual herbicide activity are applied to fields or gardens to raise certain vegetables, flowers, or other broadleaf crops, potentially devastating damage can occur.”

Well, crap. It appears gardeners not only need to know the diet of the animals that provided their garden manure, but they need to know what that diet was for the past three years.

That’s a lesson being learned on both American coasts. A Bellingham blogger reported last fall, “Aminopyralid, manufactured by Dow Chemical, and marketed as Milestone and Forefront among other brand names, is in some ways a breakthrough: it’s very potent, working with low application rates; it persists in the soil, so it doesn’t need to be used often, unlike, say Roundup; and best of all, it seems to be a relatively low risk to humans and animals either through skin contact, inhalation, or ingestion... .” He then warned, “But that same persistence and lack of reactivity—it passes through a cow’s digestive tract essentially unchanged—is bad, bad news for composters. Local dairy cows ate hay containing aminopyralid, (a local seller of compost) collected the manure and composted it, and his customers put a potent and persistent broadleaf herbicide in the soil for their tomatoes, peppers, and peas.”

It’s a valuable (and for some, costly) lesson in knowing where your food comes from; the entire cycle of your food; not just what you eat yourself, but what that food eats as well. The persistence of herbicide in the manures of animals fed with hay from fields where it was applied points out that consequences may sometimes be felt far beyond the initial causative agent.

Dow AgroScience warns of this complication on the label of their product, but there is no pro-active requirement that farmers who use the herbicide pass along this information to those who buy their hay. And there’s no requirement that those who use the hay to feed their animals then share that information with those who use those animals’ manure to fertilize their gardens. Indeed, many of those who don’t actually use the herbicide itself may not even be aware of its presence.

If you suspect your soil has been contaminated with this herbicide, several sources suggest a simple test. Prepare at least one, or several, small pots with the soil in question. Prepare several more pots with soil you know is healthy (buy a bag of potting soil at the store if need be). Plant peas in each pot. After three weeks (or when three sets of leaves appear) evaluate leaf growth. In soil containing herbicide, leaves will have a ‘cupped’ appearance (outer edges of leaf will curl inward—see image). If you see damage, then discard all contaminated soil, or use in an area where broadleaf plant growth is not desired.

“All parties need to be aware of the possibility of residual herbicide activity,” warns the North Carolina Cooperative Extension. “Hay producers should inform buyers about herbicides they have applied to their fields and provide them with a copy of the herbicide label with the restrictions. Likewise, livestock and horse owners who give or sell manure for composting or crop production should be aware of what they are feeding their livestock and horses and share that information. All parties should communicate with the end users of the hay and manure. Farmers and gardeners should ask about the herbicide history of manure, compost, hay, or grass clippings they acquire. Farmers and gardeners need to be fully informed about what they are applying to their soil because the results can be disastrous for a farm business or gardener if one of these herbicides has been applied.” (Emphasis ours.)


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Landon Otis

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Homepage, Headlines, gardening, compost, Milestone, herbicide, aminopyralid, Dow Chemical

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