The fertility of a sub-population of stallions is negatively affected by ambient climatic conditions, mediated through DNA damage

Abstract Exposure to systemic heat stress in mammals adversely affects sperm production, male fertility, and DNA integrity. To date, few studies have investigated this phenomenon in horses, particularly in industry-relevant environments. Therefore, this study examined the relationship between ambient climatic conditions and fertility within a population of commercially fertile stallions. Post-coital semen samples were collected weekly from 46 Thoroughbred stallions during the 2017 and 2018 breeding seasons (n = 804). Sperm concentration, motility and DNA damage were assessed, and fertility statistics (first cycle and per-cycle pregnancy rates) were collected. Loggers were placed in the stables and paddocks of stallions to record ambient temperature and humidity, on 3-min cycles. Our results indicate that current management regimens, involving the stabling of stallions at night, with paddock access during the day, expose stallions to highest ambient climatic conditions. Polynomial distributed lag modelling identified 18 stallions whose fertility was adversely affected by heat stress, based on correlations between fertility and ambient climatic conditions (p ≤ 0.05). Of these 18-candidate stallions, six (13% of the overall population) exhibited repeated positive relationships between ambient climatic conditions and sperm DNA damage, while repeated negative relationships between fertility and sperm DNA damage (r ≥ 0.50; p ≤ 0.05). These results suggest that heat-induced subfertility is most likely mediated via oxidative damage to the sperm DNA, and that current management strategies should be assessed to prevent exposure of stallions to maximal climatic conditions.