Mares have a natural breeding season. Increasing daylight stimulates receptor centers in the brain, which in turn trigger the production of reproductive hormones. These hormones initiate the pattern of regular periods of 'heat", or estrous, that characterize the breeding season each spring. These periods continue throughout the summer, and cease during the autumn.
By artificially increasing the amount of light - for instance, by using electric lights in a stable - it is possible to begin the breeding season earlier. This practice is prevalent in Thoroughbred studs, which try to produce the foals as near as they can to January 1st, the official birthday of all Thoroughbred racehorses.
The ideal time for a foal to be born is between May and July, when most grass is available to help the mare's milk supply. Because pregnancy in horses lasts 11 months, the best time to have the mare covered is from June through August.
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