Separating Fact from Myth: The True Value of Older Thoroughbred Broodmares Revealed

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Breeding from older thoroughbred broodmares does not mean the offspring will be of lower quality, new research findings from Japan suggest.

The results, reported in the journal PLOS ONEalso support the view that stallion quality is the primary driver of Thoroughbred performance in offspring.

The quality of the sire is well recognized as crucial in determining the racing performance of a thoroughbred horse. However, conventional wisdom holds that foals born to older mares are less likely to win high-stakes races.

The reason for this lack of racing success is not entirely clear. Some breeders and punters believe the cause is biological, with foals born to older mares facing more health problems, shorter lifespans and less resistance to stress.

It is also possible that breeders avoid mating older females with successful males, selecting inferior bulls or “cheap stallions” instead. Since breeders often select lower quality bulls as partners for older broodmares, this can result in less successful offspring.

To investigate this question, Dr. Sota Inoue from the Graduate School of Environmental Studies at Nagoya University collected information on more than 15,000 thoroughbred racehorses registered with the Japan Racing Association.

It collected data on trainer, location of training center, birth years of sire and broodmare (broodmares 16 or older considered senior), breeder, earnings, the total number of races, the number of races won, and the quality of the sire.

The quality of the father was calculated by estimating the average income index, which is the average income of his offspring over a 20-year period from 1995 to 2015. Inoue then used a statistical model to determine whether the age of the broodmare or sire quality were most related to the number of victories of their offspring.

The number of victories of horses born to older broodmares was indeed found to be lower than that of horses born to younger ones, with older broodmares producing more offspring that did not win any races.

However, the most important factor in determining a horse’s success was the quality of its male parent. After sire quality control, no relationship was found between broodmare age and horse success. In other words, the effect of maternal age was not biological, but was probably the result of the tendency of some breeders to mate older broodmares with lower quality bulls.

“The effect of maternal aging was negligible or limited, if the offspring was healthy enough to compete as a racehorse,” Inoue explains in the study. “Instead, the quality of the breeding stock, which often declines with the age of the broodmare, diminishes the performance of their offspring.”

Rumors and half-truths are common in sports that involve gambling. Inoue says the key to separating fact from myth is not just looking at the numbers, but also using statistical models.

“I did this study because there are a lot of unsubstantiated theories being spread online as facts,” Inoue says.

“I found that the age of broodmares did not directly influence the racing performance of the offspring, so it was not reliable information for racing predictions. Our results reaffirmed the importance of not be fooled by falsehoods.

Inoue’s work has been supported by the Japan Society for the Promotion of Science.

Inoue S (2022) Influence of the aging of the brood mare on the racing performance of her offspring. PLoS ONE 17(7): e0271535. https://doi.org/10.1371/journal.pone.0271535

The study, published under a Creative Commons Licensecan be read here.

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