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

Advanced paternal age in Australian Thoroughbred racehorses is linked to lower reproductive success and poorer racing performance of offspring. This study analyzed extensive breeding and performance records to demonstrate that horses sired by younger stallions are more likely to race and achieve success in stakes races compared to those sired by older stallions.

Background and Purpose

• Reproductive aging affects fertility and the health/performance of offspring in mammals.
• In humans and model organisms, older paternal age correlates with decreased sperm quality due to oxidative stress, DNA damage, and epigenetic changes.
• These biological mechanisms impacting offspring from older males have not been extensively studied in large populations of equines.
• Thoroughbred racehorses have detailed breeding and performance records, making them an excellent model to study the impact of paternal age on reproductive success and offspring outcomes at scale.

Study Design and Data

• This research is a retrospective observational study using historical data.
• Data analyzed included 35 retired stallions and 33,546 of their progeny.
• The key variable was the sire’s age at the time of conception, categorized primarily around younger (≤ 8 years) versus older (≥ 19 years) paternal ages.
• Outcome measures were reproductive success (likelihood of progeny being conceived and racing) and progeny’s racing performance, including placing and winning stakes races.

Statistical Methods

• Generalized linear mixed-effects models were used to assess associations between paternal age and outcomes.
• These models accounted for hierarchical structure of the data, potential confounders, and variability between stallions and offspring.
• Results are presented as odds ratios (OR) with 95% confidence intervals (CI) to show the strength and precision of associations.

Key Findings

• Progeny conceived when stallions were 8 years old or younger had more than double the odds of racing compared to those conceived when stallions were 19 years or older (OR=2.21, 95% CI 1.93–2.52).
• These younger-sired offspring were about 3.7 times more likely to place in a stakes race (OR=3.71, 95% CI 2.74–5.04).
• They were also approximately 3.6 times more likely to win a stakes race (OR=3.58, 95% CI 2.43–5.29).
• These significant effects demonstrate that older paternal age is associated with reduced reproductive success and diminished progeny racing performance at the population level.

Implications and Conclusions

• The study provides compelling evidence that paternal age negatively impacts both the likelihood that offspring will race and their success on the track.
• Findings extend known effects of paternal aging from humans and model organisms to equine populations, suggesting similar spermatogenic and genetic mechanisms may be involved.
• From a breeding and racing industry perspective, the data highlight the importance of considering stallion age to optimize progeny outcomes.
• Further research could investigate the underlying biological changes in sperm from aging stallions and potential interventions to mitigate these effects.