Horses are typically considered seasonal breeders whose natural breeding season occurs in the spring and summer. While this fact might be trivial to most owners, it is significant for Thoroughbred racehorse owners and breeders who want to produce offspring as early in the year as possible.

Regardless of the month in which a Thoroughbred racehorse is born, his official birthday is Jan. 1. This means that, despite technically being the same age, horses born early in the year (in February, for instance) will generally be larger than those born later in the year (e.g., in June), said Kazuyoshi Taya, DVM, PhD, of the University of Agriculture and Technology’s Cooperative Department of Veterinary Medicine Laboratory of Veterinary Physiology, in Tokyo, Japan.

“Thoroughbred breeders and owners want to get large and strong horses in the same age to entry in the classic race,” Taya added.

In a recent study, Taya and his colleagues evaluated the effects of an extended photoperiod (EP, or hours of daylight) treatment in Thoroughbred colts and fillies from winter at 1 year old to the following spring at 2 years old. Similar to previous research, the team determined that breeders can use periods of extended light exposure to stimulate testicular function in colts and ovarian function and early ovulation in fillies