A very interesting paper released today on the influence of age on racetrack outcomes...some background....
Firstly, I did my own study on this back in 2013, looking specifically at paternal/sire outcomes as a stallion aged and the data showed that as the stallion got older, he was less likely to produce superior runners. I wrote about it in International Thoroughbred here (and did a blog post abut it...hence the part II here) and it pointed to the fact that even with the best stallions, their rate of production of superior runners diminished significantly as they aged.
There was some further work in July this year (read here) where in a population of Japanese horses they studied the influence of maternal/broodmare aging on its offspring’s racing performance. They found the quality of sires was significantly associated with the offspring’s racing performance rather than the broodmare’s age itself. eg. as the mare aged, she was sent to lesser performed stallions, on average, and this was a determinant of racetrack outcomes for older mares. The paper used an Average Earnings Index to estimate sire performance (a faulty metric I think) but they concluded that it was the effect of the sire that had more influence than the age of the mare.
This paper was quite a different finding to that of Barron (1995) where she found that the average Timeform Rating of any foal out of a mare, peaks when its mother is 9 years old, after which it declines on average in subsequent foals. Similar to Barron's study, and contrary to the recent Japanese study, negative relationships between maternal age and the likelihood of winning stakes races (the highest class of races) was reported by Finocchio (1985). Additionally there was a Thesis done by Xiurui Cui under Dr Jill Stowe @UKAgriculture back in 2016. It was more about the hedonic price modeling of the age of the mare but had a similar finding - a negative correlation between dam age and performance - this can be found here.
And finally we get to the paper released today by Sharman et al. Here is a link to the paper but I will also summarise some interesting findings in the paper.
It was a large study - 41,107 dams and 2,887 sires with over 900,000 race performances by over 100,000 individual horses on British racecourses between 1996 and 2019. It is easily the largest study to look at the influence of age on outcomes
The study accounted for many of the factors that one would consider confounding, including the distance and going of the track that the offspring ran on as well as the number of runners in the race.
They found a ‘significant effect’ of maternal age on speed, with each additional year of age at conception decreasing the offspring speed by 0.017 yards per second which converts into a difference of approx 1sec/6 lengths over a race of one mile between offspring of say a 5YO and a 15YO dam.
Intriguingly, the paternal age also showed a decrease of 0.011 yards per second for every increasing year in stallion age – interesting given thoroughbred stallions play no active involvement in parental care. Overall the study points to ageing parents having a detrimental effect
Finally they suggest that epigenetic mechanisms might be at play, raising the question of whether parental age effects found in the study could cascade across further generations (e.g. you buy a mare who has old parents and her foals have a lesser chance of being a runner)
The last point is one that interests me as an explanation - epigenetic changes - where there are DNA methylation differences as a stallion (and a mare) ages which might begin to explain these outcomes. Without getting into the weeds too far, and boring the reader, DNA methylation is a major control program that modulates gene expression. A horse could 'inherit' a positive genetic variation from its sire or dam but the DNA methylation pattern modifies its expression in the offspring, so the foal never gets the 'goodies' it should have.
Aging and specifically disease is strongly correlated with changes in DNA methylation in sperm. I did once have what I suspect was first hand experience of the latter with the good Australian stallion Snippets (broodmare sire of Champion Sire Snitzel among others). The second year I had started at Arrowfield and was part of the team managing Snippets' career and coming out of winter and into the into the weeks before the 1996 covering season Snippets was quite sick. It was a bit of a drama at the time as I recall as he was coming off being relocated from TransMedia Stud (now Olly Tait's Twin Hills) and was about to stand at I believe $25,000 his highest fee in his career to date as he had just had an amazing year. We'd also lined up his biggest book of mares to cover - around 110 from memory - which was a big increase from previous years where he'd been covering around 70 (those were the days!). Snippets covered his mares in the 1996 season but it was a testament to Peter Orton, Wayne Bedggood and the team at the farm that he got through it, as he was not quite himself throughout the season (the horse had the most wonderful temperment). He seemed to have recovered well enough in 1997 and served good sized books through to his unfortunate death as a 17-year-old some six years later.
Looking back on the statistics of his career does however paint an interesting picture.
The breeding season he was sick and covered his first big book of mares resulted in 77 foals, with just 3 of them being stakes winners, none of them good enough to win at group level. It was an even more dire performance the following year with 71 foals resulting in just 1 stakes winner! In his prior and subsequent crops to these two years he had no problem producing plenty of stakes winners and at group level with his best crop numerically being his third last. It is conjecture, because we can not now be sure that Snippets' sperm DNA methlyation pattern was altered, but knowing his sickness and the effect it would have on sperm the numbers certainly point to the probability that Snippets health had an effect on his offspring.
I can't remember which book I read it in, but I do recall Federico Tesio used to make the trip to see the stallions each season before they covered his mares to check on their 'vitality'....maybe he was onto something....