Star potential remains in older Thoroughbred broodmares, findings suggest

The tendency to use lesser quality stallions over older broodmares is the most likely explanation for less successful offspring.
Photo by PapaPiper

Older Thoroughbred broodmares, long considered to produce less successful racing offspring than their younger counterparts, are more than capable of producing the next track superstar, research suggests.

A fresh Japanese study has found that while their offspring tend to be less successful on the track, the broodmares are more likely to have been bred to low quality or cheap stallions.

Nagoya University researcher Sota Inoue, writing in the journal PLOS ONE, acknowledged that maternal aging has negative influences on the development and racing performance of their offspring. “However, the mechanism by which pregnancy at old age reduces the race performance of the offspring is unknown,” he said.

Inuoe investigated two possible explanations:

  • That foals born to older mares are more likely to have muscular, skeletal, and cognitive disadvantages, which he termed direct effects; and
  • That foals born to older mares are more likely to be affected by non-physiological factors correlating with the mare’s age, such as the quality of sires, with low-quality stallions more likely to be chosen for breeding.

To test these hypotheses, the effect of broodmare age on their offspring’s racing performance was examined.

Inuoe did so by examining online data for racehorses registered to the Japan Racing Association, collected from the Japan Bloodhorse Breeders’ Association website.

In all, information on 17,885 Thoroughbred horses, born between 2001–2010, was gathered. He logged the sex, trainer, location of the training center, the birth year of the sire and broodmare, breeding farm, location of the breeding farm, earnings, the total number of races, number of races won, and the quality of the sire.

Each sire’s quality was quantified by the average earnings index (AEI), a metric calculated by the average earnings of its offspring.

Inoue’s modeling was careful to control for the effects of the stallion’s quality and age, the offspring’s sex, the trainer, and the location of the training center.

“Overall, results showed that the racing performance of horses born from older mares was lower than that of horses born from younger mares,” he wrote.

However, the modeling indicated that the quality of sires was significantly associated with the offspring’s racing performance, rather than the broodmare’s age itself.

Furthermore, the database analysis showed that older broodmares tended to be put to stallions of lesser quality.

“Therefore, the effect of maternal aging was negligible or only limited, and rather, the sire’s quality had an important influence on the offspring’s racing performance,” Inoue concluded.

“Low quality sires – or cheap stallions in other words – are likely to be chosen as partners of older bloodmares, which may have reproductive risks such as lower fertility and higher rate of miscarriages.”

He said the findings suggest that the conventional belief that racehorses born from older mares show lower performance may not always be accurate.

Discussing his research, Inoue said the age of broodmares did not directly influence the racing performance of offspring. “Thus it is not a reliable piece of information for racing predictions.”

He suggested it was the quality of sires, which tended to decrease as broodmare age increased, that lowered the performance of offspring.

Further studies are needed to fully understand the influences of maternal aging, he said.

“Breeding records including information on mating, fertility rates, embryonic death rates, and development should be analyzed, although these factors are not available in the web pages used in the present study.”

Inoue said unverified relationships, speculations, and rumors are common in sports. “Some of them are scientifically valid while others are invalid or more complex than expected.

“It would be interesting,” he said, “to examine these speculations using statistical methods — the results may reveal facts that cannot be predicted by one’s intuition.”

He says his study provides a new perspective on the effect of maternal aging on race performance of foals and the complex system of horse breeding.

“In the future, it would be possible to conduct similar studies with data sets from other countries. This extension will unveil further details of the mechanisms and potential diversity of breeding systems of racehorses.”

Inoue S (2022) Influence of broodmare aging on its offspring’s racing performance. PLoS ONE 17(7): e0271535.

The study, published under a Creative Commons License, can be read here

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