Linley Dixon has always been a farmer at heart.
She studied agriculture in high school and college before becoming a graduate student researching tomato diseases on organic farms at West Virginia University. After graduate school she followed her retired parents from Maryland to Colorado, where she and her husband, Peter, launched a farm they called Adobe House Farm, named after the passive solar home that was on the land.
The Soul of the Soil
Third in a three-part series on soil used to grow food crops.
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The first February they were in Colorado, Linley and Peter used aged horse manure, known as black gold to organic farmers, to get their tomato seedlings started. But only a few seeds germinated and those that did curled and stopped growing. Linley switched to bagged potting mix and started again.
That first year they grew enough to sell at the farmers market and for a small community supported agriculture operation. But as the tomatoes came in, a few plants growing where the compost had been dumped were misshapen and had leaves curled as if they had tomato curly top virus. Linley asked around, and many farmers in the area had been experiencing the same thing.
Because Linley had studied tomato diseases, she understood that viruses don’t appear in seedlings on a windowsill in the middle of February without a vector. Plus it was unusual that only the tomatoes grown over the composted manure pile showed those same symptoms. Linley suspected that something in the compost was the problem. With help from the extension service she tested for the virus and one of many persistent herbicides. Both tests came up negative, but often they do because they cannot detect every issue.
Meanwhile, the thistles in the farm’s region were being sprayed with a relatively new herbicide called Milestone, which is a version of aminopyralid, made by Dow to be used for invasive broadleaf weeds.
It does not degrade in plants and takes three days to pass through a grazing animal’s digestive system once treated forage is ingested. Manure may contain enough aminopyralid to cause injury to broadleaf plants including vegetables and ornamentals.
This means forage growers should warn people who use hay or manure from animals grazing pastures or feeding on grass or hay from areas treated with aminopyralid. Aminopyralid may not be used on hay intended for export outside the United States.
After checking with the farmer who had given her the manure, Linley discovered it came from horses that had ingested grass sprayed with Milestone.
Farmers in nearby Montrose, Colo., told Linley that Milestone sprayed on a windy day drifted, killing the farm’s solanaceous crops and the legumes. This forced them to cancel a CSA and lose income. She also heard that Milestone has gotten into composting systems in other states, including Vermont. But still, the spraying continues.
The Pesticide Action Network shows that in the United Kingdom those gardens and allotments contaminated by manure originating from farms where aminopyralid was sprayed have grossly deformed broadleaf vegetables. Linley also has observed about 20 home gardens and farms all over the Southwest with curly tomato leaves. The common denominator among them is that they all used organic manure from fields sprayed with Milestone. “Most of our important food crops are broadleaf plants, so it makes sense that an herbicide designed to kill broad leaf plants could harm crops,” she said. “The symptoms of herbicide carryover damage the most sensitive crops. Some farmers even report that their crops have failed.”
Even though she is an organic farmer, Linley understands some sustainable farms use an integrated pest management system that includes the use of chemicals in some circumstances. They must be used responsibly, though, which includes spraying them on non-windy mornings and not using them on crops, she said.
“But the aminopyralids are like DDT, their half life is extremely long. These herbicides simply stick around,” she said.
Dow said aminopyralid is safe to use and poses little threat, pointing out that the EPA has given the product its green chemistry award because of its ability to control weeds with just small applications.
“It can, however, pass through the systems of grazing animals unchanged and pose unintended problems for sensitive plants if manure from animals grazing treated material is composted,” the company said in a written response to Zester Daily’s inquiries.
Dow said it warns users on aminopyralid labels about these potential problems. The company advises any gardener or farmer who experiences problems with aminopyralid damage to not eat contaminated produce, even though the chemical residue from the herbicide should be so low that it wouldn’t be harmful to people or animals.
“As a general rule, if the produce is damaged, I’d disc it under as a soil amendment; and if it isn’t, I’d eat it,” a company representative wrote. “Best thing to do though may be to ask questions right at the start before accepting compost, manure (plant material, etc.), of uncertain origin and if you don’t get straight answers, test the material by using it as intended on a few of the plants you plan to grow and wait and see what happens.”
An indefinite legacy
Adobe House Farm was lucky. Linley quickly identified and diagnosed her problem and stopped using composted manure. Her garden is now growing beautifully. She is studying the area where the compost was dumped to test how long the herbicide will stick around. Right now, that’s three years and counting
Linley has switched to coffee grounds, leaves and other organic matter such as her farm’s own pasture grass clippings. She never harvests from the spot where the contaminated horse manure pile was located. But she thinks the use of aminopyralid and chemicals like it will could force farmers and gardeners to stop the age-old practice of trusting manure as a good source of fertilizer.
Top photo: Linley and Peter Dixon at Adobe House Farm. Credit: Katherine Leiner