Advanced paternal age is associated with reduced reproductive success and offspring racing performance in Australian Thoroughbred racehorses.

Reproductive ageing influences fertility and offspring fitness across mammals. In humans and model organisms, paternal age has been associated with changes in sperm quality and genomic integrity, including oxidative stress, DNA damage, and epigenetic alterations; however, these mechanisms have not been directly examined in large equine populations. Thoroughbred racehorses provide a system to investigate paternal age effects at scale using detailed breeding records. In this retrospective observational study, with data from 35 retired stallions and 33,546 progeny, we assessed associations between sire age at conception, reproductive outcomes and progeny racing performance using generalized linear mixed-effects models. Progeny conceived when stallions were ≤ 8 years were more likely to race (odds ratio [OR] = 2.21, 95% CI 1.93-2.52), place in a stakes race (OR = 3.71, 95% CI 2.74-5.04), and win a stakes race (OR = 3.58, 95% CI 2.43-5.29) than those conceived at ≥ 19 years. These findings demonstrate population-level associations between paternal age and reproductive outcomes.