Race performance of thoroughbred horses determined by the quality of the male parent rather than the age of the female, study finds

The price of racehorses can range from thousands of dollars to millions. With so much money at stake, breeders are constantly looking for ways to prevent their money from being wasted. The conventional wisdom is that foals born from older broodmares have a lower chance of winning high-stakes races.

However, the reason for this lack of racing success is not entirely clear. Some breeders and punters speculate that the cause is biological, with foals born of older broodmares facing more health problems, shorter lifespan, and lower resistance to stress. On the other hand, it is also possible that breeders may avoid mating older females with high-performing males, selecting instead lower quality sires, or "cheap stallions." Since breeders often select lower-quality sires as mates of older broodmares, this may result in less successful offspring.

To investigate this question, Dr. Sota Inoue of the Graduate School of Environmental Studies at Nagoya University collected information on over 15,000 thoroughbred racehorses registered with the Japan Racing Association. He collected data on the trainer, location of training center, birth years of both the sire and broodmare (with broodmares 16 years old or older being considered old), breeder, earnings, total number of races, the number of races won, and the quality of the sire. Sire quality was calculated by estimating the average earning index, which is the mean earnings of its offspring over a 20-year period from 1995 to 2015. Dr. Inoue then used a statistical model to investigate whether broodmare age or sire quality was most related to the number of wins of their offspring. The findings were reported in PLoS ONE.

The number of wins of horses born from older broodmares was indeed found to be lower than those of horses born from younger ones with older broodmares birthing more offspring that did not win any races. However, the most important factor in determining the success of a horse was the quality of its male parent. After controlling for the sire quality, no relationship was found between the broodmare's age and the horse's success. In other words, the effect of maternal age was not biological, but was probably a result of the tendency of some breeders to mate older broodmares with lower-quality sires.

"The effect of maternal aging was negligible or limited, if the offspring was healthy enough to compete as a racehorse," Dr. Inoue explains in the study. "Instead, the quality of sires, which often decreases as the broodmare's age increases, lowers the performance of their offspring."

Rumors and half-truths are common in sports that involve gambling. Dr. Inoue believes that the key to distinguishing fact from myth is to not only look at the numbers, but also use statistical models. "I made this study because there are a lot of unfounded theories that are being spread online as fact," Inoue says. "I found that the age of the broodmares did not directly influence the racing performance of offspring, therefore this was not a reliable piece of information for racing predictions. Our results reaffirmed the importance of not being fooled by mistruths."

Funding: This work was supported by the Japan Society for the Promotion of Science.