Characteristics and Sodium Pentobarbital Concentrations of Equine Mortality Compost Piles in the Upper Midwest.

• Equine mortality composting is effective during spring and fall in Minnesota. • Mostly large bones remained in compost piles after 6 months of composting. • Composting reduced estimated sodium pentobarbital quantities. • Physicochemical characteristics of compost were suitable for land application. Recent changes in rendering availability for chemically-euthanized animals have created a need to explore composting as an equine mortality management method. The objectives of this study were to evaluate equine mortality composting in the Upper Midwest during fall and spring, document sodium pentobarbital concentrations throughout the process, and determine nutrient content of finished compost. During each season, four horses were euthanized by intravenous administration of sodium pentobarbital. Carcasses were positioned at the center of a woodchip and shavings base and covered with a 2:1 mixture of horse stall waste and cattle feedlot waste. Data loggers were placed at 46- and 91cm depths to record pile temperatures every 8 hours. At turning (44–50 days) and trial conclusion (181–216 days), compost piles were sampled for sodium pentobarbital and nutrient composition. Piles contained large bones after 6 months of composting. Sodium pentobarbital remained detectable at trial conclusion (<0.002–1.49 mg kg¹ dry matter); however, composting reduced the estimated quantities of sodium pentobarbital by ≥94% (P ≤.001). Compost from each season met ideal land application values for organic matter (45%–48%), pH (7.3–7.6), and electrical conductivity (3.3–3.4 mm hos cm⁻¹). Low NPK and high C:N ratio (20–30) indicate compost could partially replace synthetic fertilizers when land applied. These findings suggest equine mortality composting is an effective management practice during fall and spring in the Upper Midwest. While remaining sodium pentobarbital residues were minimal, further research is needed to determine environmental implications of composting chemically-euthanized equines. [ABSTRACT FROM AUTHOR]