Estrous Cycle in the Mare (Female Horse)

The normal estrous cycle in the mare (female horse) is 21-22 days. Equine heat period displays behavioural signs of sexual receptivity to the stallion, lasts for 5 to 7 days.

During diestrus, the luteal phase that lasts for 14-15 days, the mare displays sexual rejection to the stallion.

Duration of estrus varies among individuals and also among estrous cycles of the same mare.

Long duration of estrus in the mare may be due to the fact that :

• The ovary is surrounded mostly by a serous coat and some follicles have to migrate to reach the ovulation fossa to rupture.
• The ovary is less sensitive to exogenous FSH than other species (eg. cattle and sheep), so that the preovulatory follicle requires a longer time to reach maximal size.
• The level of LH is low compared with FSH and this delays ovulation.

The intensity of behavioural estrus varies both throughout the estrus period and among individual mares at comparable stages of the period.

Old mares, mares underfed during the early part of the breeding season and during twin ovulations have a longer duration of estrus.

Symptoms of Estrus in Mare

• Restless and irritable.
• Frequently adopts the micturition posture and voids urine with repeated exposure of the clitoris by prolonged rhythmic contractions. This is known as winking of clitoris.
• Introduction of a stallion or teaser, these postures are accentuated.
• Raises the tail to one side and leans her hind quarters.
• Vulva becomes large and swollen.
• Labial folds are loose and readily open for examination.
• Vulva becomes scarlet or orange, wet, glossy and covered with a film of transparent mucus.
• Variable amount of mucoid discharge.
• If the mare is in estrus, the stallion will usually exhibit “Flehmen”.
• Vaginal mucosa is highly vascular, and thin watery mucus may accumulate in the vagina.
• During estrus cervix dilates enough to admit 2-4 fingers, during diestrus only one finger can be inserted.
• If the mare is in estrus, the stallion will usually exhibit “Flehmen”.