Mare Reproduction

Female Reproductive Anatomy The female genitalia lie in the pelvic cavity consist of: Ovary. Oviduct. Uterus. Cervix. Vagina. Vulva.

Female Reproductive Anatomy

Female Reproductive Anatomy

Female Reproductive Anatomy Ovaries

Reproductive Seasonality Physiological breeding season Ovulatory season April through September Physiological non – breeding season Anovulatory season (anestrus) December and January Transitional period Spring: February and March Fall: October and November

Reproductive Seasonality

The estrous cycle is under seasonal control, and daylight is the environmental signal for this regulation. Mares are polyestrus during the breeding season. During the reproductive season, mares will come into heat on average every 21 (18-24) days; ponies have a longer mean cycle (25 days). The oestrus lasts for about 5 (3-9) days and ovulation takes place 24-48 hours before the end of oestrus.

Estrus (heat) lasts an average of 5 to 7 days, the longest heat period of any domestic animal. Estrus is characterized by receptivity to the stallion. A mare showing classical estrous behavior will adopt a urination stance – squatting with legs spread apart and tail raised. She will urinate small volumes frequently and expose her clitoris by everting her vulva (winking).

If a stallion nears, she may lean into him. Most mares cease estrous behavior within 24 to 48 hours following ovulation.

Reproductive activity in the spring is stimulated by increasing day length Mechanism involves alteration of hormone secretion by the pineal gland and hypothalamus Pineal gland: melatonin Hypothalamus: GnRH Artificial lighting can be used to hasten the onset of the ovulatory season

Influence of the length of day light on the ovary function in mares Photoreceptors in the eyes increasing day light Influence of the length of day light on the ovary function in mares decreasing day light neurotransmission Pineal Melatonin Melatonin Hypothalamus portal system GnRH GnRH Anterior pituitary blood circulation Gonadotropine Gonadotropine Ovaries

Minimal follicular development Physiological Non – Breeding Season (Winter Anestrus / Anovulatory Season) Minimal follicular development GnRH, FSH and LH secretion are low small, firm ovaries Follicles < 15 mm in diameter Mares may exhibit estrous behavior (heat)

Transitional Period Increased day length stimulates GnRH secretion from the hypothalamus GnRH stimulates FSH and LH secretion FSH induces follicular development Estrogen produced by developing follicles causes the mare to exhibit irregular and / or prolonged estrus When LH levels have increased, the first ovulation of the year occurs

Transitional Period 1 to 3 “waves” of follicular growth and regression Irregular or prolonged estrus Enlarged ovaries with multiple “clustered” follicles

Physiological Breeding Season 21 day inter-ovulatory interval (estrous cycle) Estrus (heat): 5 – 7 days Ovulation: 24 to 48 hours prior to end of heat Diestrus: 14 – 16 days

Follicular Dynamics 1 to 2 waves of follicles emerge during each estrous cycle Each wave consists of a dominant follicle and several subordinate follicles

Ovulation LH surge induces final maturation of the follicle and ovulation The ovulated follicle then develops into a corpus luetum (CL)

Corpus Luteum (CL) Function The primary function of the CL is the secretion of progesterone, which is necessary for the maintenance of pregnancy

Corpus Luteum Regression Nonpregnant mares must regress their CL in order to initiate another estrous cycle The duration of CL function in the nonpregnant mare is 14 to 16 days Prostaglandin F2alpha is secreted in a pulsatile pattern from the uterus on days 14 to 16 after ovulation, which eliminates function of the CL