The Estrous Cycle

A complete review of mare endocrinology is given in Chapter 7. An important factor to remember when evaluating apparent cycle abnormalities is the normal variation in length of estrus. Estrus length in normal cycling mares can vary at least from 2 to 12 days. Estrus length is generally repeatable within mares, and is longer at the beginning and end of the breeding season (the peak of the breeding season being the first day of summer, June 21 in the Northern Hemisphere). In some mares, estrus becomes so short in midseason that it may be missed if teasing is only performed sporadically. Owners may report that their mare was showing good heat in April and May but that they haven’t seen her in heat in June.

Teasing is a major determinant of apparent cyclicity. Individual teasing of mares with an active stallion is the best method of heat detection. To adequately detect cyclical changes in behavior, teasing should be performed thoroughly at least three times weekly, and the mare’s behavior scored by a knowledgeable individual. “Silent heat” may occur in mares that have normal ovarian function. The meaning of this term is relative, indicating only that the observer did not recognize estrous behavior. Knowledge of the estrous and diestrous reactions of each individual mare is crucial in heat detection; one mare in heat may be less demonstrative than another mare in diestrus. Some mares may show signs of estrus immediately on contact with the stallion; others—especially young or timid mares—may require teasing for 3 to 4 minutes before they respond. A mare that shows no change in behavior at all during her cycle, as assessed by an experienced observer using good heat detection methods, may present a challenge. Such a mare may need to have the reproductive tract examined regularly by transrectal palpation and ultrasonography to detect estrus (see later discussion). However, assessing the willingness of the mare to allow mounting by a stallion may reveal that she is receptive to him, even though teasing evoked no signs of estrus. Mares that do not show good signs of estrus to a stallion may begin to show estrous behavior if they are bred by natural cover, during estrus, on a few occasions.

Estrous behavior in mares does not necessarily indicate follicular activity. Estrous behavior can be seen in mares in seasonal anestrus, in mares that have been ovariectomized, and in mares with gonadal dysgenesis. This is because sexual behavior in the mare is largely regulated by progesterone; that is, estrous behavior is suppressed by progesterone. In the absence of progesterone, even the small amount of estrogen produced by the adrenals is sufficient to cause the mare to show some signs of estrus. Further stimulation with estrogen does, however, increase the intensity of estrous behavior. This is reflected in the finding that mares in true estrus (during the follicular phase of the cycle) have fewer negative reactions to mounting or intromission than do seasonally anestrous or ovariectomized mares.¹

The length of diestrus is more repeatable among mares than is the length of estrus, at 15 ± 2 days.¹ When evaluating heat detection records, repeated diestrus intervals of normal length, interspersed with estrous intervals, are a good indication that corpora lutea are forming and being lysed normally. The formation of corpora lutea in turn suggests that normal follicle growth and ovulation are occurring.

Hormone analysis can also be helpful in defining the pattern of cyclicity in a mare; progesterone concentrations are of most value. Reflecting teasing records, a pattern of high progesterone (increasing to >4 ng/ml) for approximately 14 days followed by low progesterone (<1 ng/ml) for 3 or more days is strongly indicative of normal cyclicity.

Ovarian and Uterine Characteristics of Normal Cyclicity

In assessing the normality of follicle growth and ovulation, the large variation in follicle size at ovulation should be recognized. The size of the follicle the day before ovulation is detected is commonly 35 to 45 mm in diameter, but mares can ovulate much smaller follicles (less than 30 mm), or larger follicles, with normal fertility. Follicle size at ovulation is often repeatable for a given mare. Small follicle size at ovulation may be a cause of apparent infertility because, unless this tendency is known, the mare may not be bred at the appropriate time. When double ovulations occur, the follicles ovulate at a smaller size than for single ovulations. Follicle size at ovulation also decreases toward the middle of the breeding season (July and August). Numerous large follicles and corpora lutea are normally present on the ovaries of pregnant mares, especially between 30 and 120 days. These are sometimes mistaken for ovarian pathology.

Transrectal palpation and ultrasonography are indispensable tools in the evaluation of the estrous cycle. Familiarity with the normal appearance of the ovary and uterus on ultrasonographic examination throughout the cycle is a great help when assessing possible abnormalities.² Multiple follicles may be present on the ovaries at any stage of the cycle. During estrus, the dominant preovulatory follicle is usually identifiable at approximately 25 mm diameter, when it is about 5 mm larger than other follicles on the ovary. In some mares, however, multiple follicles appear to grow to a large size (>30 mm) from which one or two ovulate; this is associated with normal fertility. On ultrasonographic examination, follicles usually are round and should be echolucent (black) in appearance. In the day or so before ovulation, infrequent small echodensities may be seen within the follicle, and the follicle may become less round, reflecting a palpable loss of tone. Immediately before ovulation, follicles may appear pointed on ultrasound examination, as they apparently protrude toward the ovulation fossa.

On the first day after ovulation, the collapsed follicle may be difficult to define ultrasonographically. The only finding may be that the follicle seen previously is gone; however, a thick echodense line or mass is usually visible within the ovary. At this time, a depression may be palpable on the ovary, and the ovary may be sensitive to palpation. The normally developing corpus luteum (CL) has two major forms on ultrasonographic examination.² Over the first few days after ovulation, the CL may become more and more apparent as an echodense structure within the ovary; then, after about 7 days, it may become less dense and less easily delineated. Alternatively, the ovulated follicle may fill with clotted blood over 1 to 2 days. This corpus hemorrhagicum has a typical ultrasonographic appearance of a round structure with an echodense rim and an echolucent interior, crossed with a lattice of echodense fibers. The echolucent area decreases in size over the next few days as the echodense rim thickens; after 5 or more days this structure is indistinguishable from the CL that did not have an interior clot. A normal corpus hemorrhagicum is typically no larger than was the original follicle; a larger structure with similar appearance on ultrasonography may be a hematoma (see later discussion).

Uterine changes detectable on ultrasonographic examination are also helpful in staging the cycle in mares with questionable estrous behavior. The uterus in estrus becomes edematous. This is characterized on palpation as a heaviness and lack of tone, and ultrasonographically by presence of echolucent areas of fluid within the uterine folds, giving a “wagon wheel” or “orange slice” effect to the cross-sectional uterine image. In diestrus, the uterus is homogenous in appearance on ultrasonographic examination and tends to be more tightly circular in cross section, reflecting increased tone. Changes in palpable cervical tone reflect those of the uterus but may be more influenced by individual variation between mares; cyclical ultrasonographic changes in the cervix are not obvious.