Time to put non-pregnant broodmares under lights

Breeders of all types of performance horses generally wish to have foals born as early as possible in the yearly calendar, even though Mother Nature typically tries to give non-pregnant mares the winter off. The winter anestrus (when mares do not have normal estrous cycles and do not ovulate), allows mares to only begin breeding in warm weather, and have foals born the following spring in warm weather, due to their very long gestation period of over 11 months.

In the wild, it is important for mares to avoid mating in the cold months in order to have foals born when food resources are more plentiful and the chances of life-threatening cold exposure is minimized. However, horse breeders with access to heated indoor barn space often wish to move the breeding season, and thus, the following year’s foaling season, earlier in the calendar year in order to have the most well-developed youngsters when they go into training as yearlings or early 2-year-olds.

Increasing daylight length stimulates ovarian activity in mares. By manipulating photoperiod in non-pregnant mares, it is possible to change the beginning of the natural breeding season from around May 1 to mid-February. Since the “universal birthday” for most breeds of horse is Jan. 1, this allows foals born in January (from mares bred the previous February) to be almost a full year old on their first “official” birthday, while foals born to mares under normal ambient light may be only be 8 or 9 months old on their first Jan. 1 birthday.

The most common management method to achieve an earlier first ovulation of the year is using an artificial photoperiod that simulates daylight lengths that would normally be encountered in spring. Mares should be put under lights in late November or early December to stimulate follicular development and ovulation by early February. It usually takes 60 to 70 days of artificial photoperiod to stimulate the first ovulation. The most common technique is to provide 16 hours of light and allow eight hours of darkness. It is not necessary to keep mares in lighted stalls continuously for 16 hours of constant artificial lighting, although that is an acceptable method. Providing additional hours of artificial light beginning at dusk, to supplement natural light up to 16 hours also works well. The use of timing devices to control the lighting system will provide the right amount of light with less effort.

Follicular development on the mare’s ovary will usually be stimulated if mares are housed under 10 or more foot-candles of artificial light. This intensity of light can be provided by one 200-watt incandescent light bulb in a 12-by-12-foot stall. Mares should either be locked into lighted stalls during dark hours, or be provided adequate lighting in attached outdoor runs if mares are allowed to wander outside during dark hours.

A less restrictive method to provide a controlled amount of supplemental light to horses, while allowing movement into non-lighted areas, is the Equilume light mask. The mask is fitted to the mare’s head, which allows for housing in any environment, including pasture. Research has found that this method is as effective as stalling under lights.

Some may worry that artificial lighting starting in late fall to jump start mares’ reproductive cycles early in the following year is “fooling Mother Nature.”. However, many years of research have supported the efficacy of this management technique, and also its safety, as long as good common sense, like keeping mares in a warm barn for early foaling, is practiced.

Kevin H. Kline, Ph.D., is a professor of Animal Sciences, University of Illinois Department of Animal Sciences.