The influence of mare numbers, ejaculation frequency and month on the fertility of Thoroughbred stallions.
Authors: Allen W R, Wilsher S
Journal: Equine veterinary journal
Summary
# Editorial Summary: Stallion Fertility Factors in Thoroughbred Breeding Reproductive performance varies considerably amongst Thoroughbred stallions, yet little was known about the specific management and seasonal factors driving these differences. Allen and Wilsher analysed breeding records from 31 stallions covering 3034 mares across 4851 matings during the 2010season on East Anglian studfarms, correlating daily mating data with ultrasound-confirmed pregnancy rates and investigating the influence of mare book size, calendar month, daily ejaculation frequency and recent ejaculation history. Overall fertility averaged 59.6% per mating, though individual stallion performance ranged dramatically from 19.0% to 80.1%, with a notable decline in fertility during June and July; importantly, neither the size of a stallion's mare book (15–161 mares) nor the total seasonal workload nor daily mating frequency significantly predicted pregnancy rates, though some individuals did show sensitivity to cumulative ejaculations over the preceding three days. The findings suggest that most Thoroughbred stallions can sustain acceptable fertility even with large books, yet the substantial inter-stallion variation and the identification of a minority cohort (5 of 31) with suboptimal fertility (≤50%) indicates that individual stallion management—particularly around seasonal timing and potentially recovery intervals between ejaculations—merits closer attention from studfarm managers and veterinarians seeking to optimise reproductive outcomes.
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Practical Takeaways
- •Monitor individual stallion fertility through pregnancy rates rather than assuming workload capacity; large books do not necessarily compromise fertility in most TB stallions
- •Plan breeding schedules to prioritize matings in earlier months (February–May) when pregnancy rates are significantly higher than late season
- •Identify poor-fertility stallions early using pregnancy rate data to guide management decisions and inform owners about expected conception outcomes
Key Findings
- •Overall per mating pregnancy rate was 59.6% across 31 stallions, but ranged widely from 19.0% to 80.1% between individuals
- •Pregnancy rates were significantly reduced in June and July compared to earlier breeding months
- •Mare book size (15–161 mares per stallion) and daily mating frequency were not related to per mating pregnancy rates
- •Five of 31 stallions (16%) showed per mating pregnancy rates ≤50%, suggesting individual variation in semen quality or fertility factors