The Mare’s Estrous Cycle

– an overview of the hormonal relationships that go to make up the reproductive cycle in the mare

Terminology

“Estrous” (œstrous in many parts of the world outside North America) refers to the entire cycle;
“Estrus” (œstrus) refers to the “heat” stage of that cycle when the mare is receptive to the stallion’s advances;
“Diestrus” (diœstrus) refers to the period in between the estrus phases when the mare is not receptive to the stallion;
“Anestrus” (anœstrus) refers to the compete absence of estrus;
The mare is a “seasonally polyestrus” (polyœstrus) animal, meaning that she undergoes regular estrus cycles during a portion of the year (late spring, summer and early fall) and none at others (winter). This is nature’s way of preventing the arrival of a foal during bad weather.

These cycles are controlled by the mare’s hormones, which in turn respond to an increase or decrease in daylight duration with the onset of spring or fall, which affects the pineal gland. This study of hormonal activity is known as endocrinology.

It is important to understand that there is a closely linked feedback system between many of the reproductive hormones present in the mare which will alter the level or presence of some hormones as levels of other different hormones increase or decrease. This means that artificially altering a single hormone will be likely to have an effect on one or more of the other hormones. The same can be said of natural hormonal changes – whether they are happening in a correct manner or not. Many of these hormonal changes do occur naturally, but when something becomes unbalanced either naturally or artificially, we can see estrous cycle problems develop in the mare.

Hormones Active during the Estrous Cycle of the Mare:

• Gonadotropin Releasing Hormone (GnRH)
  • Secreted by the hypothalamus of the brain, GnRH has a controlling influence over other reproductive hormones.
  • Early in the year stimulation of the pineal gland by light – either natural or artificial – causes a reduction of melatonin secretion, which in turn allows GnRH to be secreted by the hypothalamus, thereby stimulating the production of other hormones (FSH, LH). The exact connection between decreased melatonin production and increased GnRH levels is not understood.
  • In anestrus mares GnRH is released in a pulsatile manner with long period between secretions, resulting in undetectable blood levels of the hormone.
  • In estrous mares secretion is continuous with additional pulses ranging from every 2 hours in diestrus mares, to twice hourly during estrus.
  • The use of exogenous GnRH using either a subcutaneous mini-pump or an implant to facilitate the onset of early estrus by shortening the transitional phase has not been shown to be successful on a widespread scale and is not therefore generally recommended.
• Follicle stimulating hormone (FSH) and Luteinizing hormone (LH)
  • Both the gonadotropins FSH and LH are produced in the pituitary gland and are regulated in a similar manner by GnRH, although there is also a controlling influence by other hormones that differs between FSH and LH.
  • LH concentrations are lowest during the mid-luteal phase of estrous, rising only a few days before the onset of estrus to a peak usually on the day of, or shortly after ovulation, to then drop to previous levels over the next few days.
  • The duration of secretion of LH in the mare and its associated ovulatory surge is considered to be longer than in most other animals.
  • LH as its name suggests, is the hormone that has a controlling interest in the development and maintenance of the CL. It does however also work in close harmony with FSH to achieve final follicular maturation and ovulation.
  • FSH is in contrast to LH, thought to follow a bi-modal secretion pattern, although some researchers suggest that this twin-peak effect is seen only during spring and early summer; and that it adopts a single-peak effect in later summer and fall.
  • In the bi-modal pattern the initial FSH surge starts late in estrus and peaks early in diestrus, dropping slightly and with the second surge starting in mid-diestrus peaking in late diestrus.
  • In the uni-modal pattern seen later in the breeding season, researchers suggest that the initial post-ovulatory surge of FSH is absent.
  • Despite measured increases in FSH it has been suggested that follicular growth itself follows a singular developmental path commencing in mid-diestrus, with a mechanism that selects a single follicle for optimum growth and subsequent ovulation.
• Estrogens
  • There are several forms of estrogen occurring in the mare. In the non-pregnant mare estradiol is the most active; in the pregnant mare there are several, but the most important to note is estrone sulphate.
  • Estradiol in the estrus mare is secreted by the follicle and serves several purposes. Its presence is associated with the behavioural displays common to estrus;
  • Estradiol also causes the relaxation of the cervix during estrus – with it becoming more relaxed;
  • Estrogen present during estrus promotes uterine immune response;
  • Estrogen presence has a negative impact on FSH, but a positive impact on LH;
  • It is thought that there is a link between sufficient estradiol levels and early pregnancy maintenance, perhaps as a result of an effect of estradiol on uterine condition;
  • In the pregnant mare estrone sulphate is secreted by the feto-placental unit (FPU). Levels rise gradually until about day 70 post-conception ovulation, when they rise dramatically and remain elevated until about day 210, when they start to slowly drop again until shortly before foaling;
  • The presence of estrone sulphate in the mare’s blood from about day 70 of pregnancy onwards can be used as a reliable indicator of fetal viability as its levels drop rapidly following fetal failure.
• Progesterone
  • Progesterone is the hormone that, among other things, in non-pregnant mares prevents the display of estrus. It is also, in part, responsible for increasing uterine and cervical tone in the diestrus mare;
  • Progesterone is secreted by the Corpus Luteum (CL) that is formed in the evacuated follicle left behind after the mare has ovulated;
  • Luteal Progesterone has a positive impact on FSH and a negative impact on LH.
• Prostaglandin
  • Pulsatile secretion of Prostaglandin F2α by the lining of the uterus commences if pregnancy is not detected at about day 14 post-ovulation.
  • The presence of PGF2α causes the “lysing” of the CL and an almost immediate drop in circulating levels of progesterone, which permits the mare to start displaying estrus.
  • This pulsatile secretion actually continues for several more days.
  • PGF2α also causes a contraction of smooth muscle – which includes the uterus.
• Inhibin
  • Inhibin is a follicular hormone that in addition to estrogen quietens the positive impact that progesterone has on FSH follicular maturation and selection.

A Brief Sequential Overview of the Regular Estrous Cycle

Key Features:

• The entire estrous cycle from ovulation to ovulation takes approximately 21 days, with only a couple of days possible variation in the normal cycle;
• Ovulation of a follicle occurs, releasing the oocyte for potential fertilization;
• The cavity left behind on the ovary by the evacuated follicle fills rapidly with blood and in some cases forms a Corpus Hemorrhagicum prior to creation of luteal tissue to become a Corpus Luteum (CL) as a result of the presence of LH;
• By about 5 days after ovulation, the CL is fully functional and secreting progesterone. This is also known as the luteal phase;
• FSH action early in diestrus may produce a mid-cycle follicle that will sometimes ovulate, but more usually regresses;
• Around day 13 post-ovulation the endometrium of the uterus secretes PGF2α which causes the destruction (“lycing”) of the CL, which will then permit the onset of estrus behaviour;
• FSH activity in late diestrus will have caused the selection of a dominant follicle (or possibly two) which under the influence of increasing estrogen levels early in estrus will develop to reach ovulatory status approximately 21 days after the previous ovulation occurs.
• The mare will display receptive behaviour (estrus) towards the stallion for 5-7 days and will ovulate in the last 24-48 hours of that display period.

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