A mare will show signs of heat (sexual acceptance of the stallion) about four to seven days of her approximately 21-day-long cycle. In between the heat periods, she is not receptive to a stallion. To check this, you need to tease her with a stallion, something that stallion owners will do as a daily or every-other-day routine to mares. A mare will urinate, swat, lift her tail, and show behavior consistent with receptivity to the stallion when she is in heat. However, these reactions are variable, so breeding farms will use a veterinarian to palpate the mare's reproductive tract or use an ultrasound machine to view the ovary to better pinpoint the time she releases an egg (occurs the latter part of a heat cycle).
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