Heat stress is detrimental to sperm production and male fertility, but the extent to which ambient temperatures affect stallion fertility has not been examined in a field setting relevant to the horse breeding industry. This study aimed to investigate how temperature and humidity interact with stallion fertility.
Dismount samples were collected weekly from 45 Thoroughbred stallions during the 2017 (n=486) and 2018 (n=318) breeding seasons. Samples were diluted with EquiPlus semen extender, and sperm concentration and motilities were recorded before snap freezing and fixing for DNA damage and morphological assessments, respectively. Stallion management data were collected, including per-cycle conception (PCC) rates and libido scores. DNA was assessed via a combined sperm chromatin dispersion (SCD) and oxidative DNA damage assay (8-OHDG). Temperature and humidity loggers were installed in stables and paddocks.
Stallions susceptible to heat-stress (n=13) were identified based on strong correlations between ambient temperature and PCC rates. Within this group, almost all stallions (n=12) demonstrated strong relationships between temperature/humidity variables and sperm DNA damage (R2≥0.50); of these, seven stallions displayed significant inverse relationships between DNA damage and PCC rates. Stepwise linear regression was performed with DNA damage (SCD) as the response, and normal morphology, paddock temperature (PT), stable temperature (ST) and humidity (SH; recorded one(-1), two(-2) and three weeks preceding(-3)) as predictors (R2=0.75), such that;
SCD = -0.89 + (-0.64×Normal Morphology) + (0.02×Max Day PT-3) + (-0.06×Min Night ST) + (0.05×Max Day ST-1) + (0.02×Min Day SH-1) + (-0.04×Min Night ST-2)
In conclusion, fertility in heat-stress susceptible stallions is compromised by high ambient temperatures and these effects are likely to be mediated by damage to sperm DNA cargo. Interestingly, libido and normal morphology were also influenced by ST and SH (R2=0.79 and R2=0.71, respectively), suggesting environmental conditions affect circulating testosterone levels in addition to direct effects on sperm quality.