Inducing earlier cycling of mares using artificial light program (A review)

Abstract

Mares are seasonally polyestrus, that is, they have multiple heat cycles during certain seasons of the year. Most mares do not cycle during the shorter days of fall, winter and early spring. Mares enter their ovulatory cycle in the spring in response to increased daylength. Although the natural breeding season for mares is in spring, Thoroughbred breeders try to gain early foal, in early summer. Horse breeders (especially Thoroughbred and trotter breeders) are looking for ways to shorten winter anestrus, therefore they have found this can be done by the use of artificial light to extend sunlight. Research suggests that extending the day length (from 16 to 18 hours) by adding light starting in the late afternoon is better than turning the lights on earlier in the morning and shortening the night length. Using the artificial light program, mares can be in oestrus and fertilize earlier in the breeding season, therefore breeders can breed larger, stronger two-years-old foals with a greater chance to win a competition in the horse racing industry. The authors summarize the physiological background of the applied artificial light program.