horses grazing in pasture with trees

Your horse might be compostable—even if you’ve used chemicals to humanely euthanize him.

According to results from a new study, composted horse carcass piles meet ideal standards for organic matter and electrical conductivity with favorable nitrogen, potassium, phosphorus, and carbon ratios after only six months, making them good candidates for replacing synthetic fertilizers in crop fields. And because the concentrations of sodium pentobarbital—the drug that causes respiratory arrest in horses under sedation—reduce by at least 94% during that time, their impacts on the environment would likely be minimal, said Hannah L. Lochner, MS, from the University of Minnesota’s Department of Animal Science, in Saint Paul.

“We found that composting ended up reducing the concentrations of sodium pentobarbital within the compost pile, when the pile was properly constructed and managed,” she said. “There are concerns for the environment and for wildlife scavenging with regard to (this chemical), but when the piles are managed correctly, those risks are actually reduced.”  

While the American Veterinary Medical Association has approved multiple ways to humanely end a horse’s life—including gunshot and captive bolt, which require no chemicals—most veterinarians opt for a barbiturate overdose, mainly using sodium pentobarbital. The drug works quickly and is cost-efficient, and its effects are less traumatic for horse owners, Lochner said.

Unfortunately, though, sodium pentobarbital can poison other animals that consume the carcass, and it can pollute water for decades after burial, she said. Incineration is biosecure, but few owners choose cremation due to the expense and impracticality. Composting might be the most environmentally friendly solution, but fewer than a fourth of veterinarians suggest it, and only 12% of owners consider it.

The reason for composting’s unpopularity, said co-author Mark Hutchinson, MS, from the University of Maine’s Cooperative Extension, in Orono, might be because owners see horses as part of their family. “Disposal of an older horse becomes much more personal than for a cow, chicken, turkey, or hog,” he said.

Promising Research Points Toward Compost as an Option

Previously, scientists had little knowledge about what happens on an environmental level when horses are composted, the researchers said.

Lochner, Hutchinson, and their team investigated the effects of composting eight carcasses of horses humanely euthanized with sodium pentobarbital due to terminal medical conditions. Four of them—all about 25 years old—were euthanized in September, and the other four—all about 21 years old—were euthanized the following May. The bodies were laid on a thick layer of wood chips covered by a thin layer of pine shavings, and they were covered by a mixture of two-thirds horse bedding manure and one-third cattle bedding manure. All the piles were located in the Upper Midwest, in Minnesota. The team turned the piles over after about 50 days.

In both spring and fall, the piles reached internal critical temperatures of 104-149 degrees F within a few days of composting, Lochner said. These temperatures reflect the microbial activity going on inside the pile, she explained.

The piles reestablished those temperatures within three to four days following turning, Lochner said. After this heating phase, the piles entered what’s known as a mesophilic phase, where the internal temperature was closer to the surrounding outdoor temperature. The researchers could see some flesh, hair, hide, and bone when turning the piles, she said, but by the end of the six-month period, only large bones and a bit of hide were visible.

By the end of the six months, sodium pentobarbital levels had reduced to below toxicity levels, meaning if a dog came across the pile and found the bones, he would not consume a lethal amount of the chemical, she said.

Game cameras showed that no animals—including wildlife such as coyotes and deer—interfered with the compost piles over the six-month period, Lochner said. This might be because they were adequately covered with manure mixture, which acts as a biofilter, controlling odor.

Overall, the fall composts spent more time in the mesophilic phase—which is also the maturing phase—than the spring piles, she said. But both seasons contributed to good chemical ratios, such as carbon to nitrogen, possibly because the horses were composted on a bed of wood chips and shavings.

“Compost can improve soil health by increasing soil organic matter and water-holding capacity, replenishing and supporting microbial communities, and providing slow-release nutrients to plants, thus improving soil fertility and tilth,” Lochner and her team reported.

The composts met the ideal organic content matter of 40% to 60% for most kinds of plant management, she said. Their pH values varied but were essentially neutral and within most plants’ tolerance levels.

As for electrical conductivity (an indicator of the soluble salt content), the scientists found levels well below maximum, which contributes to good crop management, Lochner said.

Chemically speaking, the composts were low in nitrogen, phosphorus, and potassium compared to synthetic fertilizers, she said. Even so, they represent “a valuable nutrient source” and can create slow-release nutrient stores in soil, which could reduce the long-term need for synthetic fertilizers.

Sodium pentobarbital concentrations dropped to about 6% of what they were before composting, said Hutchinson. The other 94% was broken down into constituents that “don’t raise any red flags” for water and ground pollution, he said.

This is reassuring, but it still raises unanswered questions, Hutchinson said. “What’s the fate of the sodium pentobarbital at that point in time?” he asked. “You know, it’s pretty small numbers spread over a pretty large area. So I don’t have a lot of concerns, and most people don’t. But we still don’t have good research on that aspect of it.”

In general, composting has both environmental and economically sustainable advantages, said Lochner.

Hutchinson agrees. “You end up with a beneficial end product rather than, you know, rendering the carcass, which has benefits but is also costly,” he said. “But there are significant advantages compared to burying it or putting it in a hole someplace, where there is no long-term benefit of that protein and that carbon.”

The study, Characteristics and Sodium Pentobarbital Concentrations of Equine Mortality Compost Piles in the Upper Midwest, first appeared in the Journal of Equine Veterinary Science in July, 2022.