Tradition in many breeds holds that all horses have the same birthday: January 1. But when it comes to feeding young horses, it might be better to do so according to each horse’s individual birth date, a Japanese research team recently concluded.

Because foals go through seasonal growth periods—slower growth in winter, much faster “compensatory” growth in spring—they have varying energy needs throughout the year. Even though the weather changes and actual seasons of the calendar year are the same for all foals on the same farm, the seasonal growth period for each foal can be quite different depending on the animal’s date of birth. And the effects of this seasonal growth variation could be quite significant in Thoroughbred racehorses, which go through more dramatic changes, sometimes growing very fast, said Takeshi Miyake, PhD (Agric), of the Comparative Agricultural Sciences Department at Kyoto University.

Proper nutritional management of growing horses during their specific compensatory growth periods could have a critical influence on the horse’s growth potential, as well as its resistance to osteochondritis dissecans and other juvenile osteochondral conditions, Miyake said.

Miyake and his colleagues developed mathematical equations to calculate growth curves for Japanese Thoroughbreds based on the horses’ birth dates. To create the equations, they compiled more than 15,000 measurements from nearly 500 colts and fillies over a 10-year period. Among the most important measurements were wither height and total body weight, they said.

Their equation suggested that nutritional needs can vary significantly from one foal to another depending on its birth date. That’s because, as they found, the “meeting point” between the slow growth period of winter and the fast growth period of spring is different from horse to horse, Miyake said. Even though that meeting point always occurs in the winter-to-spring transition period, that critical point might be several weeks apart, from horse to horse. The farther apart the horses’ birthdays, the more different their growth periods were, he said.

“The combined knowledge of both body weight and wither height would be useful for detection of the unbalance between weight and height which may cause the critical growth disorders on young horses, especially for the horses born in June,” the researchers stated.

“The individual percentile curves of body weight and withers height for each horse based on his/her birthday would be a useful diagnostic tool for careful feeding management of young Japanese Thoroughbred horses during compensatory growth periods,” they said.

The study, "An Approach of Estimating Individual Growth Curves for Young Thoroughbred Horses Based on Their Birthdays," was published in the Journal of Equine Science