Thoroughbred breeders experience undue pressure to achieve pregnancies close to the commencement of the breeding season, as yearling sales price and racing success are strongly affected by age. Periods of subfertility in stallions yield major cost and welfare concerns. The purpose of this study was to investigate environmental factors affecting stallion fertility. Dismount samples (n=486) were collected weekly from 45 individual stallions, across four commercial stud farms. Samples were diluted (2:1, extender:semen), sperm concentration and motilities were recorded using a haemocytometer and iSperm™ device, then fixed in 2% PFA for morphological assessment. Stallion management data was collected, including per-cycle conception (PCC) rates, and libido scores (rated 0-5), and temperature and humidity loggers were installed in stables. A sub-population of nine stallions demonstrating susceptibility to stable temperature (ST) and humidity (SH) were identified based on correlations with PCC rates, and investigated further. Stepwise linear regression was performed using PCC as the response, and total motility, abnormal head morphology, libido score, ST and SH (recorded the week of, one week preceding (-1) and two weeks preceding (-2)) as predictors (R2= 0.79), such that;
PCC = 3.21 + (-0.01 × Total Motility) + (-0.01 × Head Abnormality) + (0.12 × Libido) + (-0.13 × Max Night ST) + (-0.01 × Max Day ST) + (-0.04 × Min Day ST) + (0.03 × Min Night SH) + (-0.01 × Min Day ST-1) + (0.01 × Min Day SH-1) + (-0.04 × Min Day ST-2) + (-0.01 × Max Night SH-2) + (0.02 × Max Day SH-2).
Interestingly, libido and normal morphology were influenced by ST and SH (R2= 0.79 and R2= 0.71, respectively), suggesting environmental conditions affect circulating testosterone levels; a hormone vital for normal spermatogenesis. Sub-fertility is a multifaceted issue, and this study demonstrates that non-targeted, comprehensive approaches are warranted for fertility investigations.