Top image: Researchers composted 13 cattle with a combined weight of about 3,232 kilograms in a single on-farm windrow. Photo courtesy of Maine Department of Environmental Protection.
Mark A. King and Paula Luu
Composting has long been recognized as an effective method for managing livestock mortalities. The process offers environmental benefits that include nutrient recycling, pathogen reduction, and odor control. Questions remain, however, about how pharmaceutical compounds used during animal euthanasia behave within composting systems. A newly released study – led by Mark King of the Maine Department of Environmental Protection (MDEP), alongside Mark Hedrich and Kasia Szymanska from the Maine Department of Agriculture, Conservation and Forestry (MDACF), and Meghan Flanagan of Creative Veterinary Solutions – found that two commonly used anesthetic agents, ketamine and xylazine, degrade substantially during active composting of livestock carcasses.
MDEP’s Division of Materials Management, evaluated the fate of these compounds in a mortality composting windrow constructed after a livestock depopulation event. Over a 13-week composting period, concentrations of both drugs declined by more than 80%. The findings suggest that properly managed composting systems can significantly reduce pharmaceutical residues present in carcasses.
The research adds to a growing body of evidence supporting composting as a viable and environmentally protective option for livestock mortality management, particularly when chemical agents are used during euthanasia.
Concerns About Pharmaceutical Residues
Humane euthanasia is a routine practice in livestock management, used when animals suffer severe injury or illness and during emergency depopulation events such as disease outbreaks or contamination incidents. Veterinarians often administer sedative or anesthetic drugs before physical euthanasia to ensure animal welfare and reduce stress.
Two commonly used drugs are xylazine, a sedative widely used in large animal medicine, and ketamine, an anesthetic known for its rapid onset and short duration. While effective, these compounds raise questions about their persistence in carcasses after disposal and the potential for environmental contamination. Residues may also pose a risk of secondary toxicosis if scavenging wildlife or domestic animals consume contaminated tissues.
Traditional disposal options have limitations. Burial can threaten groundwater in areas with shallow water tables. Rendering infrastructure has declined in many regions and certain compounds may persist through the rendering process. Incineration can address pharmaceutical concerns but is often costly and raises air quality considerations.
Composting has emerged as a widely used alternative. The process relies on microbial activity to convert organic materials into stable organic matter while generating heat that accelerates decomposition. Despite its growing use, relatively little research has examined how composting affects veterinary pharmaceuticals used during euthanasia.
Composting a Depopulation Event
The King Et al. study was conducted following a cattle depopulation event linked to per and polyfluoroalkyl substances (PFAS) contamination in livestock feed. The animals exceeded the state Action Level for PFOS in beef and could not enter the food supply in a timely or economically feasible manner.
Researchers composted 13 cattle with a combined weight of about 3,232 kilograms in a single on-farm windrow. Prior to euthanasia the animals received intramuscular injections of ketamine and xylazine for sedation, followed by dispatch using a penetrating captive bolt.
The windrow was constructed according to USDA livestock mortality composting protocols using layered organic materials to support microbial activity and decomposition. A base of sawdust shavings and haylage absorbed liquids released during breakdown while providing structural support. Carcasses were placed on the base and covered with a mixture of haylage and manure that supplied additional carbon and nitrogen while helping retain heat and control odors.
Photo caption: Newly constructed windrow using USDA livestock mortality composting protocols.
Researchers monitored the windrow over a 13-week composting period. Composite samples were collected at multiple intervals and temperature readings were recorded daily at two depths to track microbial activity. The pile was turned four times to maintain aeration and support decomposition. Environmental conditions such as rainfall and ambient temperature were also recorded, and the windrow was regularly inspected for odors, vector activity, and leachate.
No leachate was observed leaving the windrow during the study period.
Significant Reductions in Drug Concentrations
Laboratory analysis of compost samples showed steady declines in both pharmaceutical compounds during the composting period. Initial concentrations measured when the windrow was constructed were approximately 198 parts per billion for xylazine and 66 parts per billion for ketamine. By the end of the 13-week composting period concentrations had declined to about 24 parts per billion for xylazine and 10 parts per billion for ketamine.
These results represent reductions of roughly 88% for xylazine and 85% for ketamine. The largest decreases occurred during the first eight weeks of composting when microbial activity and temperatures were highest. The findings indicate that composting conditions can significantly reduce the concentration of these compounds over time.
Reduction in xylazine and ketamine concentrations during 13 weeks of composting.
Mechanisms of Degradation
Researchers attribute the reduction of ketamine and xylazine to several interacting processes within the compost environment. One is sorption, in which drug molecules bind to organic matter in the compost matrix. Humic substances in compost contain negatively charged functional groups that attract positively charged pharmaceutical compounds.
Once bound to organic particles, these compounds become more accessible to microbial degradation. Compost microorganisms produce enzymes that break down complex molecules, particularly under thermophilic conditions that support intense microbial activity. During this process organic compounds are gradually transformed into simpler substances such as carbon dioxide and ammonium.
Chemical reactions also play a role. Oxidation and other transformations can alter the molecular structure of compounds like xylazine and ketamine, making them more susceptible to microbial breakdown. Together these biological and chemical processes reduce pharmaceutical concentrations within the compost pile.
The findings also point to the need for further research on the fate of veterinary drugs in compost systems. Future studies could examine whether small amounts of compounds migrate into soil beneath windrows or explore how different compost recipes influence degradation rates.
Read the full King, et. al report here.
The 8th International Symposium on Animal Mortality Management, to be held June 29 to July 2, 2026 in Stevens Point, Wisconsin, will bring together researchers, regulators, and practitioners to share advances in mortality management. Established in 2005, the triennial symposium highlights new research, technologies, and education on disinfection, decontamination, depopulation, and disposal. The 2026 full symposium program is available online.
Mark A. King is an Organics Management Specialist at the Maine Department of Environmental Protection and a Director of the Maine Compost School. King has more than three decades of experience in composting, facility development, and organics recycling. He provides technical assistance on compost facility design and operations and serves as a USDA subject matter expert in mortality composting and disaster debris management. Paula Luu is a circular economy expert and the Managing Director of BioCycle.
