10/21/2026
A study published in Scientific Reports in 2026 examined whether a Thoroughbred stallion’s age at conception is associated with his fertility and the racing success of his offspring.
Although research in humans and other mammals has linked paternal ageing to changes in s***m quality and genetic integrity, relatively little was known about how sire age relates to racing performance in Thoroughbred offspring.
Breeders also face a practical dilemma because establishing a stallion’s value through his offspring’s racing results takes years. By the time his success is recognised, he may already be well into his breeding career.
The researchers analysed historical breeding and racing records from 35 commercially significant Australian Thoroughbred stallions and 33,546 offspring.
They followed the same stallions through different stages of their breeding careers, examining live-foal production, whether offspring reached the racetrack, and whether those that raced became winners, stakes performers, stakes winners or Group winners.
Advancing sire age was consistently associated with lower fertility and reduced progeny racing success. Estimated seasonal fertility, measured as live foals per mare bred, declined from 76.5% in stallions aged eight or younger to 66.8% in those aged 19 or older.
The estimated proportion of foals that reached the racetrack also fell, from 81.2% to 66.7%.
Among offspring that raced, the estimated proportion winning at least one race declined from 71.2% in the youngest sire category to 57.2% in the oldest. Differences were also evident at higher levels of competition, with estimated stakes-winner proportions falling from 4.3% to 1.3% and Group-winner proportions declining from 2.0% to 0.5%.
While focused on Thoroughbreds, the study raises important questions for all horse breeders about how a stallion’s advancing age may influence the fertility and future performance of his offspring.