Researchers Study Light's Impact on Young Thoroughbreds

Researchers Study Light's Impact on Young Thoroughbreds

An extended photoperiod appears to have variety of effects, from encouraging coat shedding to increasing bone mineral density.

Photo: Anne M. Eberhardt/The Horse

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.

The extended photoperiod stimulates the brain, specifically the hypothalamus and the pituitary, which secretes gonadotropins. Gonadotropins include prolactin, a follicle-stimulating hormone (FSH), and luteinizing hormone (LH). These hormones stimulate testis and ovary function, encouraging sperm production in males and ovulation in females.

In natural light conditions, horses typically ovulate no earlier than April. However, mares receiving EP treatments at the Hikada Training and Research Center in Japan as early as December 10 began ovulating two to four months sooner than control mares.

“This effect is significant for breeders to get early conception in mares,” Taya said.

But that wasn’t all the researchers learned.

They determined that EP treatments increased the secretion of prolactin, a hormone that triggers milk production and aids in udder development. Prolactin also helps encourage calcium absorption from the intestine, prevent gastric ulcers, and signal when to shed the hair coat. Furthermore, Taya uncoverved prolactin’s anxiolytic (antianxiety) effect, suggesting it’s an important hormone in the brain function during stressful conditions.

Further, because EP treatments activate colts’ and fillies’ gonadal function, they also increase testosterone and estradiol-17β secretion. Both testosterone and estradiol-17β are hormones are have been linked to muscle strength and increased bone mineral density. Encouraing greater levels of testosterone and estradiol-17β reduces the risk of young Thoroughbreds in training developing motor organ or training diseases, the team said.

“Our previous (research) demonstrated that the fat thickness is lower in the EP-treated male yearling than the control, whereas the fat-free body weight was higher in the EP treated male yearling,” Taya added.

There are no differences in female yearlings in either fat thickness or fat-free body weight, he added. This suggests that the EP treatment increased circulating testosterone, thereby increasing muscle volume in male yearlings.

“Since extended photoperiod treatment-induced changes in the yearlings were within the physiological range, and the method is safe and simple, the treatment may be an effective technique in horse husbandry,” Taya said.

The study, “Effects of an extended photoperiod on gonadal function and condition of hair coats in Thoroughbred colts and fillies,” was published in the Journal of Equine Science

About the Author

Katie Navarra

Katie Navarra has worked as a freelance writer since 2001. A lifelong horse lover, she owns and enjoys competing a dun Quarter Horse mare.

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