Mares ovulate approx. at 21-days intervals during the breeding season. The oestrus (commonly known as a “mare in heat” or “mare in season”) is a phase of sexual receptivity to the stallion, and it lasts approx. 6.5 days, with a range from 4.5 to 9 days. The remaining 14 -15 days of the cycle is dioestrus or the luteal phase, a period of sexual inactivity – (the mare will not stand to be served by the stallion).
During the transitional phases, i.e. winter/spring and summer/autumn, cycles become irregular, often prolonged and sometimes unfinished with ovulation (autumn anovulatory/haemorrhagic follicles).
These conditions are easy to observe in mares or horse populations (e.g. the Konik horse (Poland), the Icelandic Horse (Iceland) the Mustang (USA) or the Brumby (Australia), that free roam in large pastures / paddocks, in natural light conditions with no access to stables or artificial light. In mares kept in stables and under artificial light, we can observe oestrus cycles throughout the whole year, but the fertility of winter cycles is questionable.
Pictured – herd of Konik Horses (Poland)
Ovulation occurs usually 24-48 hours before the end of oestrus, but the precise detection of the ovulation is challenging without an ultrasound scanner or a sensitive hand, well-trained in palpation. The precise detection of the ovulation is the critical point of the clinical evaluation, particularly when artificial insemination is used to breed the mare.
During oestrus, mares demonstrate specific behavioural patterns. However, these should be assessed in the presence of a stallion with good libido. Teasing is a standard procedure on Thoroughbred studs, where natural service is still used.
Specific symptoms of mare’s oestrus behaviour include:
Raised and arched tail, sometimes deviated to one side
‘Winking’ rhythmic flaring of vulva and exposing the clitoris
Urination of small amounts of urine
Squatting on the hind limbs
The hind limbs wide-based stance (hind legs spread apart)
Calm behaviour toward the stallion, and/or staff
Leaning with buttock to the fence/wall
Actively searching for a stallion
The intensity of the oestrus behaviour varies between individuals and can be inhibited e.g. in the presence of a dominant mare in the herd, a foal at foot or in the course of possible endocrine disorders.
Picture 1 – Teasing stallion showing good libido
Picture 2 – Introduction of the mare to the teasing stallion
Picture 3: Calm behaviour of the mare towards the teasing stallion, turning the buttock towards the stallion.
Picture 4a : Clear winking (The tail was held aside for clarity of the photo)
Picture 4b : Winking and lifting tail to the side.
Picture 5: Urinating of small amounts of Urine and squatting of the hindlegs