Mares are seasonally polyestrus, meaning they come into heat multiple times a year but only at certain times of the year. For mares in the northern hemisphere, those are times with increased day length as they normally only cycle from March to October. But since the birthdate of many horse breeds is artificially set at January 1 for aging purposes, the earlier in the year the foal is born, the better for showing and performance. So the goal is to get a mare to come into heat and get pregnant as early in the year as possible so the foal will be born as close to the first of the year as possible. It is difficult for a two-year old horse born in May to compete with a two-year old born in February because that three months makes a lot of difference in maturity, size and athletic ability at that age.
The best method to get these mares to cycle early is to expose them to light throughout the winter and trick their hormonal system into thinking the days are actually longer than they really are. So beginning December 1, most brood mares are placed in the barn at dusk and the lights in the barn are turned on so the mares will get 16 hours light per day during the winter and be ready to cycle in the spring. And although this is effective, it means you have to stall mares longer than normal, which means extra labor. Also, stalled horses tend to colic more than horses at pasture and most horses are generally happier at pasture. Dr. Barbara Murphy from Ireland has developed a face mask for the horse to wear that provides a short wavelength of blue light that mimics sunlight, so the horse can get extra light needed for cycling but can remain in the pasture. If you have brood mares that you want to get pregnant early but do not want to stall them, ask your vet about this blue light-producing face mask.
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