Temporal Trends in Equine Sperm Motility and Semen Volume: A Retrospective Analysis from a Single UK Breeding Facility

The equine industry preferentially selects sires based on pedigree, performance, and conformation, with little concern given to fertility. Increasing evidence supports the theory of geographic-sensitive declines in a range of semen quality parameters, yet the horse is underrepresented within this field. Data presented here retrospectively investigates trends in semen quality from a population of stallions at a single UK breeding facility (from 2001 to 2020). Data on stallion sperm motility (10,686 ejaculates, 984 stallions) and semen volume (11,122 ejaculates, 1,030 stallions) were collected from records during the years 2020 and 2021. Data were analyzed as isolated variables in a linear mixed model (REML). Fixed effects included significant covariates (year of collection, age, and abstinence period). Random effects included stallion and sample numbers. Overall trends indicated that motility has declined over the past 20 years (p < 0.001; overall decline: 12.19%). Motility declined similarly in both prime and senescent stallions, confirming trends are not age-specific. Trends in volume (p < 0.001) varied over time but typically increased (5.70 mL overall; 0.28 mL annually). Results suggest stallions could be at risk of perturbed reproductive health and function in the future, with serious implications for the economic status of breeding stallions and the health and welfare of breeding stock.