THE MARE

Michelle LeBlanc, Cheryl Lopate, Derek Knottenbelt and Reg Pascoe

REPRODUCTIVE ANATOMY OF THE FEMALE GENITAL TRACT

A thorough understanding of the mare’s reproductive anatomy is needed to differentiate normal from abnormal, and to identify structures during reproductive examinations. In this section important anatomical considerations are reviewed. The main structures of the breeding organs of the mare are shown in Fig. 5.1.

Fig. 5.1 Reproductive organs of the mare: (A) left lateral view; (B) viewed from above; (C) detail of the falopian tube region and the relationship of the horn of the uterus to the broad ligament.

The perineum is the area that includes the anus, vulva and the adjacent skin (usually hairless) under the tail. The normal conformation of the perineum prevents the ingress of air and bacteria into the genital tract. This area is very important because of its protective role for the genital tract and its implications in some forms of infertility in particular. It is also a common site for injury at parturition. The anatomic arrangement of the perineum, including its length relative to the pelvic bones and its angle relative to the vertical plane, are important features for the fertility of the mare; for example, mares that experience bouts of infertility frequently have a flat-topped croup with a tail setting that is level with the sacral iliac joint and a sunken anus (Fig. 5.2). Alterations of angle and length have been used to determine a Caslick score¹ (see p. 185) which is used to provide an index of the need to perform a vulvoplasty operation (Caslick’s operation, see p. 184). The normal/ideal anatomic arrangement is that the vulva should be vertical (or at least less than 10% from the vertical) and that more than 80% of its length (from dorsal commissure top ventral commissure) should lie below the level of the ischial tuberosities (Figs 5.3, 5.4). Variations in either the relative length below the ischium or an increase in the angle of declination (or both) results in a tendency for the vulva to be drawn forwards into the perineum below the anus, especially during later pregnancy (Fig. 5.5). This combination results in an increased chance of aspiration of air into the vagina and a higher Caslick score.

Fig. 5.2 Flat-crouped mare.

Fig. 5.3 Evaluating perineal conformation in a normal mare.

Fig. 5.4 Evaluating perineal conformation in a mare with poor conformation.

Fig. 5.5 An example of very poor vulvar conformation. The upper half of the vulva slopes forward and is almost horizontal. The anus is drawn forward.

Three seals protect the genital tract:

• The vulvar seal (created by close contact between the vulvar lips or labiae and the skin and muscles of the perineum).

• The vestibulovaginal seal (formed by the posterior vagina, the pillars of the hymen and the floor of the pelvic girdle).

• The cervix, which acts as a final barrier for the uterus.

Any deficiency in one or more of these ‘seals’ has implications for the reproductive efficiency of the mare. During a normal reproductive examination these seals are broken and air allowed into the vagina. The presence of air in the vagina allows bacteria to be carried into the anterior reproductive tract. The presence of the air itself is responsible for a typical inflammatory response with blood vessel engorgement. When any of the three seals are disrupted (either through trauma, faulty acquired or congenitally poor perineal conformation, or poor body condition) air may be intermittently or consistently aspirated into the vagina and the resulting contamination and infection can be a serious cause of infertility. The vulvar and vestibulovaginal seals usually restrict the ingress of clinically significant bacteria. The vagina and uterus should be free of significant infection although some incidental bacteria are commonly found.²

• Simply gently parting the lips of the vulva and listening for aspiration of air into the vagina can test the integrity of the vestibulovaginal seal.

• Normally the vestibulovaginal seal will maintain an airtight seal when the lips of vulva are parted in this way. In-rushing air suggests poor vestibulovaginal seal efficiency.

Adhesions and damage are important causes of infertility (see p. 174). Examination of the cervix usually involves manual palpation per vaginum and visualization using a speculum or endoscope (see p. 136).

Uterus and broad ligaments

The uterus is a T-shaped structure. The two uterine horns and body are located entirely within the caudal part of the abdominal cavity. Distension and movement of the intestines and bladder as well as pregnancy influence the position of the reproductive tract.³ The uterine horns range from 20 to 25 cm in length. The uterine body averages 18–20 cm in length.

The uterus is suspended within the abdomen by two large broad ligaments that extend along the lateral sublumbar and lateral pelvic walls from the third or fourth lumbar vertebra to the level of the fourth sacral vertebra. These ligaments serve as attachments to the body wall and support blood vessels, lymphatic vessels, and nerves. The broad ligaments also contain a large amount of smooth muscle that is continuous with the outer longitudinal muscular layer of the uterus and oviducts. Although the broad ligaments are a continuous sheet they are commonly divided into three nondemarcated areas:

• The mesometrium, attaching to the uterus.

• The mesosalpinx, attaching to the oviducts.

• The mesovarium, attaching to the ovaries.

Cystic remnants of mesonephric tubules and ducts (epoöphoron and paraoöphoron) occur commonly in the mesovarium and mesosalpinx.

The ovaries

The ovaries are kidney-shaped with a very prominent depression (the ovulation fossa) on the free or ventral border. The ovary has two surfaces (lateral and medial), two borders (dorsal or attached and ventral or free), and two poles (cranial and caudal). These are used in the description of normal and pathological changes.

The mare’s ovaries are larger than those of other domestic species and vary considerably in size during the year depending on the number and size of follicles on their surface. During reproductive quiescence, the ovaries may be the size of walnuts. During the cyclic season when there is a 50-mm follicle on the ovary, it may be the size of a large orange. On cut section, there may be follicles in different stages of development and atresia. There may be a developing or regressing corpus luteum along with remnants of corpora lutea of previous cycles; 10–12 days later, the appearance of the same ovary will have completely changed.

• It is extremely important, therefore, to record all palpable and ultrasonographic findings during reproductive examinations (see p. 134).

The ovaries can move freely within the abdomen, thus their orientation and location are variable, making it difficult to identify poles and surfaces. The mesovarium attaches at the dorsal aspect of the ovary and extends for a considerable distance over the medial and lateral surfaces. Blood vessels and nerves reach the ovaries through the broad ligaments and enter each ovary at the dorsal convex border and spread over the lateral and medial surfaces.

The equine ovary is unique in its internal structure. Unlike other organs that have the cortical area as the outer portion or external layer and the medulla in the center, the adult equine ovary has the opposite arrangement. The medullary (vascular) zone is superficial and the cortical zone that contains the oocytes and follicles is partially within the interior of the gland. The cortex reaches the surface only at the depression (ovulation fossa) on the free border, which is the only area from which normal ovulation occurs. The corpus luteum does not project from the greater surface of the ovary as in other species and is therefore not palpable during rectal examination.

Oviduct

The oviducts are tortuous tubes within the mesosalpinx, with a total length of some 20–30 cm.

The oviduct is divided into:

• Infundibulum.

• Ampulla.

• Isthmus.

At ovulation, ova drop into the expansive funnel-shaped infundibulum that covers the ovulation fossa. They travel quickly through the infundibulum to the ampulla where fertilization and early development of an ovum occurs. The fertilized ovum then travels through the narrow isthmus and enters the uterine horn at the uterotubal junction. Unfertilized ova normally are retained for a considerable time in the oviduct before degeneration occurs.⁴ Unfertilized ova rarely enter the uterus.

PHYSIOLOGY

PUBERTY

Puberty is the onset of reproductive activity. There are considerable changes that are associated with this event, relating to the physical stature and the behavior of both mares and stallions. Little is understood about the mechanisms for the onset of puberty but maturation and onset of activity in the hypothalamus, pituitary and ovarian (or testicular) tissues may control its onset. A late foal (May–June, northern hemisphere) may reach puberty at an earlier age than an early foal (January–February, northern hemisphere). In any case, the onset of reproductive activity is timed to ensure that parturition occurs at a favorable time of the year for survival of the offspring. However, there are physiological and pathological causes of delay or total inhibition of the onset of puberty.

The onset of puberty usually occurs, and is currently presumed to result, from a combination of endocrinological events within the hypothalamus, the pituitary gland, and the gonads (Fig. 5.7). The onset of puberty is well recognized as a primary reproductive event and occurs at around 12–24 months of age,⁵ but this can be significantly affected by several factors, including:

Fig. 5.7 Model for endocrine control of puberty. A minus sign (−) indicates inhibitory effects of estradiol; a plus sign (+) signifies a positive effect. E₂, estradiol; LH, luteinizing hormone.

• Age (and timing of birth within the breeding season).

• Photoperiod (daylight hours). This is likely to be a significant event in the onset of puberty in horses, but this area has been poorly researched. Prolonged daylight hours result in a delayed onset of puberty; the best stimulus to puberty is the natural fluctuations in daylight hours.⁶ This contrasts with the effect of photoperiod in mares and stallions of mature breeding age (see p. 156).

• Nutritional status (body condition score) and growth rate. Although there are no data relating to the effects of nutritional status on the onset of puberty, this does occur in other species and there is no reason to suppose that it would not be similar in the horse. In ruminants, nutritional deprivation usually results in delayed onset of puberty. Whether precocious puberty occurs in response to high energy and rapid growth and weight gain in horses is uncertain. Most breeders are not driven to encourage early onset of puberty in mares in particular and prefer to rely on natural events.⁷

• Contact with other horses of breeding age (pheromone-governed). Typically, the effects of pheromones may be altered by the endocrine status of the animal and it may be that these factors have little overall influence on the onset of puberty itself.

THE NORMAL ESTROUS CYCLE AND SEASONALITY

The estrous cycle is a synchronized interplay of anatomic, endocrine, and behavioral events resulting in ovulation. Despite the high level of stud farm management and veterinary attention they receive, horses have an inherently low reproductive efficiency. A few highly significant factors contribute to this problem. They include:

• A disparity between the breeding season arbitrarily imposed by racing and performance industries and the physiologic breeding season of mares.

• Failure to select for fertility, including retention of breeding stock with heritable defects that reduce fertility (e.g. bad vulvar confirmation, cryptorchidism).

• Breeding geriatric animals.

• Prohibition of artificial insemination (in some breeds only and in particular the thoroughbred).

It is essential to understand basic equine reproductive physiology to manage normal mares for maximum fertility and to treat abnormal mares. There are wide variations in events within the cycle, particularly related to season of the year. These must be put in perspective and, if possible, used to best advantage.

Mares are regarded as ‘long-day, seasonally polyestrous breeders’, meaning that they exhibit a distinct breeding season characteristically during the spring, summer, and early autumn months. During this period, normal nonpregnant mares show repeated estrous cycles lasting about 21 days. Estrus lasts for between 4 and 6 days. During this time one or more follicles mature and the mare is receptive to the stallion. Estrus is followed by 16–17 days of diestrus, when a corpus luteum is functional and the mare rejects the stallion.

During winter, most mares pass through a period of anestrus or sexual inactivity, when neither follicles nor corpora lutea are present. The response to the stallion is neutral (being neither receptive nor rejective). Day length is the primary factor controlling this seasonal ovarian activity.^8,⁹ Long day length (15–16 hours), such as occurs during the summer, stimulates ovarian activity. The effects of day length are mediated negatively by melatonin secretion from the pineal gland within the brain. Prolonged periods of high melatonin secretion during the short winter days suppress gonadotropin-releasing hormone release from the hypothalamus. Increasing day length in spring causes shorter periods of high melatonin production, allowing an increase in the frequency and amplitude of pulsatile gonadotropin-releasing hormone release.

Seasonal effects on ovarian cycles can be divided into three phases:

1. The ovulatory phase: the period from first ovulation in the spring until the last ovulation in the autumn.

2. Anestrus: the period of ovarian inactivity during the winter months.

3. Transition: the period of irregular or prolonged estrus receptivity that occurs in early spring or late autumn.

During the spring transition period, the ovaries develop numerous follicles of varying sizes which grow and regress until finally one follicle progresses to ovulation.

In both northern and southern hemispheres, 75–80% of mares demonstrate seasonally polyestrous behavior, whereas 20–25% have estrous cycles all year. The percentage of mares that cycle throughout the year increases nearer the equator and year-round cyclicity is more common in Arabian mares.¹⁰

Anestrus

In response to the diminished photoperiod of winter and other related factors governing seasonality, the anestrus mare is best described as sexually dormant.

• The ovaries are small, smooth, and firm, with no palpable follicular activity or functional luteal tissue.

Without the stimulus of estrogens and progesterone that accompany cyclicity, the uterus becomes atonic and thin-walled.

• Uterine changes that can be seen microscopically include glandular atrophy and compaction of the stroma resulting from the absence of edema.

The cervix is similarly flaccid and difficult to palpate. The cervix, seen through a speculum, is pale, dry, and often relaxed, sometimes to the point of appearing open. The vaginal tract also is quite pale and devoid of obvious secretions.

• Distension of the vagina with air (such as follows manual examination of the vagina or the use of a vaginal speculum) produces little hyperemia or engorgement of superficial blood vessels.

During anestrus, the endocrinological functions that govern cycles essentially are shut down. As a result of a shortened photoperiod, small and infrequent pulses of gonadotropin-releasing hormone from the hypothalamus lead to baseline levels of plasma luteinizing hormone concentrations. Baseline concentrations of plasma follicle-stimulating hormone remain relatively high but fluctuate randomly during anestrus, presumably as a result of the lack of any negative feedback effect from ovarian inhibin and estrogen.¹¹

• Plasma progesterone concentrations are less than 1 ng/ml because there are no corpora lutea in the ovaries.

The behavior patterns of seasonal anestrus are less specific and less predictable than endocrine function. Most mares are passive to mildly resistant in the presence of a stallion. It is not unusual, however, for a mare in deep anestrus to be sexually receptive to the point where she accepts the stallion at any time. In the absence of follicular development, this behavior seems paradoxical. It is possible that minute amounts of steroid hormones produced by the adrenal glands could be responsible for this. In addition, the cervix is partially to completely relaxed in mares in anestrus.

Anestrous mares behave similarly to ovariectomized mares because they are under similar endocrine control. Very small doses of exogenous estrogens cause sexual receptivity in both. For this reason, spayed (ovariectomized) mares make excellent teasers for semen collection because they can be predictably induced into estrous behavior with small doses of estrogenic hormones.

Transition

The transition phase frequently coincides with the time when owners are eager to breed mares for early foals. Managing transitional mares is a major concern for practitioners and stud managers during the spring months.

In most mares, the changeover from anestrus to a fully functional ovulatory phase is gradual. This transition is characterized by the re-establishment of endocrine function, erratic sexual behavior, and follicle development without accompanying ovulation. Increasing follicular activity causes the ovaries to enlarge considerably in comparison to their size during anestrus. There is a noticeable change in consistency as multiple small follicles begin to grow (Fig. 5.8). A springy resilience is evident on deep palpation, in contrast to the dense, firm consistency noted during the anovulatory phase.

Fig. 5.8 Normal ovary (spring transition ovary). Note the numerous small atretic follicles with a few medium-sized follicles developing.

A large variability is noted in transitional mares in the number of the various types of follicles palpated. Commonly, many small (10–15 mm) follicles are clustered on the ovarian surface, suggesting bunches of grapes. These small structures remain firm and are barely distinguishable in consistency from the ovarian stroma. As the season progresses, most mares develop one to two follicles that slowly progress to 25–30 mm but remain firm and then slowly regress. Mares in transition eventually develop follicles greater than 35 mm in diameter. Although these follicles can grow to a large size, initially they regress without ovulating.

• Lack of estrogen synthesis appears to be the underlying defect in these anovulatory follicles.

In pony mares, three to four follicles greater than 30 mm develop sequentially during transition over a period of several weeks. The first and second follicles usually regress, whereas the third or fourth follicle will undergo the first ovulation of the season. Estrogen production is the hallmark of the follicles that successfully ovulate.¹² Most of the time, the first follicle of the year that ovulates is accompanied by uterine edema visualized on ultrasonography (see p. 217).

When follicles begin to develop on the ovaries early in spring, little physical change is detectable in the tubular reproductive tract. The uterus stays thin-walled and flaccid, lacking stimulation because of relatively low levels of ovarian hormones. As follicular activity increases, the uterus becomes more edematous, probably as a result of increasing estrogen concentrations.

The increase in day length during spring leads to an increase in both the amplitude and the frequency of pulses of gonadotropin-releasing hormone from the hypothalamus.¹¹

Baseline plasma follicle-stimulating hormone concentrations rise during the early stage of the transition period but then fall steadily during the 15–20 days before the first ovulation of the breeding season, presumably because of an increasing production of inhibin by developing follicles. Plasma luteinizing hormone concentrations increase slowly and steadily throughout the transition phase, with minor pulses that coincide with the pulses of gonadotropin-releasing hormone. Concentrations rise more steeply during the few days immediately preceding the first estrus, eventually reaching a typical peak at or soon after ovulation.

Plasma progesterone concentrations are less than 1 ng/ml until after ovulation, when they rise sharply. Plasma estrogen concentrations also remain low during anestrus but then rise coincidentally with the wave of follicular growth that precedes the onset of the first ovulation.¹³

• Behavior patterns during transition are erratic.

• Mares may show either no signs of estrus behavior, or constant to erratic estrous behavior.

• Mares should not be bred during the transition phase as excessive breeding during this period may result in endometritis, especially in older, pluriparous mares.

Prolonged periods of full sexual receptivity are usual as follicular development progresses. It is particularly helpful to managers to determine the difference between transitional mares showing strong estrous behavior without ovulation and normally cycling mares. Unnecessary breeding during the early season can be avoided by careful reproductive tract palpation and ultrasonography. The benefits of conserving the stallion and avoiding contamination of mares during this unproductive stage of the cycle are obvious. Some transitional mares remain passive, whereas others respond to the teaser with no correlation to ovarian status, showing receptivity and resistance in no definable pattern.

The fall transition period after the ovulatory phase of the cycle receives little attention from owners or clinicians, because the breeding season does not correspond with this period of waning reproductive function. The decreasing photoperiod in the autumn has the reverse effect on the mare to the increasing photoperiod in the spring. Behavior and ovulation become more erratic as the ovulatory season nears its end.

After the last ovulation, it is not uncommon for a follicle to develop to a large size and then fail to ovulate or regress. These so-called autumn follicles have been mistakenly referred to as ‘cystic follicles’. They are not pathologic and disappear spontaneously, often weeks to months later.

The ovulatory phase

During the ovulatory phase, mares establish cycles which, compared with those in females of other domestic species, are models of inconsistency. Acceptance of these inconsistencies as variations of the normal limits is the only reasonable way to approach broodmare management.

• Examples of these inconsistencies include mares ‘normally’ ovulating without showing estrous behavior, showing signs of estrus without ovulating, undergoing prolongation of the lifespan of the corpus luteum for a couple of months, or splitting heats with 8–10-day inter-ovulatory intervals.

The estrous cycle of the mare is defined as the interval from one ovulation to the subsequent ovulation when ovulation is accompanied by behavioral estrus and/or plasma progesterone concentrations below 1 ng/ml. Progesterone concentrations are included in the definition because ovulation may occur during the middle of the cycle (diestrus ovulation) when progesterone concentrations are high. Estrus is the period of sexual receptivity. Diestrus is the period from the end of one estrus to the beginning of the next estrus, characterized by the formation of a functional corpus luteum.

The duration of the individual components and the length of the total estrous cycle vary greatly (Table 5.1). The greatest variability is at the beginning and end of the ovulatory phase. Therefore the duration of estrus is shortest during the peak of the ovulatory season and corresponds with the peak of fertility. Breeding mares at the optimal time early in the imposed breeding season require closer management than breeding mares at the peak of the physiologic breeding season. For example, it can be assumed that ovulation occurs 24 hours before the end of estrus and that sperm is viable for 48 hours within the mare’s reproductive tract. Without palpation for follicular development, if breeding commences on the second day of estrus, three breedings are necessary to adequately expose mares with an 8–9-day estrus. Mares with a 4-day estrus need only be bred once. Due to this variability, if palpation is not or cannot be used, mares are covered on the second day and then every other day until estrus has ended. This illustrates the value of accurate palpation in avoiding unnecessary stallion services.

Table 5.1

Mean length of components of the estrous cycle

CYCLIC EVENTS OF THE OVULATORY PHASE

Three main groups of hormones are involved in control of the estrous cycle. The ‘brain hormones’ (melatonin and gonadotropin-releasing hormone) convert external stimuli into direct stimulation of the pituitary gland. The ‘pituitary hormones’ [the gonadotropins (follicle-stimulating hormone, luteinizing hormone), prolactin, and oxytocin] exert a direct trophic action on the ovaries, uterus, and other parts of the genital tract. The ‘genital or sex hormones’ [estrogen, progesterone, inhibin, and prostaglandin F₂_α (PGF₂_α)] are secreted in response to stimulation by pituitary hormones and control functional changes in the genital tract and behavioral changes in the animal. They feed back positively and negatively on hypothalamic and pituitary hormone secretion rates (Fig. 5.9).¹⁴

Fig. 5.9 Hormonal control of the estrous cycle in the mare. AP, anterior pituitary; LH, luteinizing hormone; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone.

Hypothalamic control

Gonadotropin-releasing hormone is secreted by cells in the hypothalamus and is released in brief pulses. After gonadotropin-releasing hormone reaches the pituitary gland via a portal venous system, it stimulates the secretion and release of both follicle-stimulating hormone and luteinizing hormone. The frequency of the gonadotropin-releasing hormone pulses is mediated by melatonin release. A low frequency occurs during anestrus when melatonin release is high because of the shortened day length. The frequency of gonadotropin-releasing hormone release is also reduced during diestrus as a result of negative feedback effects exerted by progesterone. The frequency of the pulses of gonadotropin-releasing hormone controls which gonadotropin is released from the anterior pituitary gland.

• High-frequency pulses of gonadotropin-releasing hormone stimulate luteinizing hormone release.

• Low-frequency pulses of gonadotropin-releasing hormone cause follicle-stimulating hormone release.¹⁵

Pituitary function

The pituitary hormones (follicle-stimulating hormone and luteinizing hormone) are responsible for the control of follicular growth and ovulation in mares:

• Follicle-stimulating hormone stimulates the initial growth of follicles in the ovaries during diestrus.

• Luteinizing hormone stimulates maturation of the follicles, maturation of the oocytes within the follicles, and ovulation during estrus.

During the physiologic breeding season of mares, follicle-stimulating hormone concentrations peak twice during each cycle at about 10–11-day intervals (Fig. 5.10). Follicle-stimulating hormone secretion is stimulated by increased day length and suppressed by inhibin secreted by developing follicles. The first peak of follicle-stimulating hormone concentrations occurs near the end of estrus. This peak coincides with the luteinizing hormone peak at or soon after ovulation. The second follicle-stimulating hormone surge is seen in mid-diestrus, when follicular activity is at its lowest. It is theorized that this surge is responsible for initiating the wave of follicular growth that provides the ovulatory follicle during the ensuing estrus. Follicles stimulated after the first surge of follicle-stimulating hormone reach the luteinizing-hormone-dependent stage between days 5 and 7 of the estrous cycle and become atretic if luteinizing hormone is not available.¹⁶

Fig. 5.10 Hormone changes occurring during the normal 21-day estrous cycle in the mare (the arrows depict ovulation). LH, luteinizing hormone; FSH, follicle-stimulating hormone; PGF₂_α, prostaglandin F₂_α.

Luteinizing hormone

Plasma concentrations of luteinizing hormone are low between days 6 and 15 (Fig. 5.10) after ovulation because of the negative feedback action of progesterone on the hypothalamus, causing the suppression of gonadotropin-releasing hormone. Blood concentrations start to rise near the beginning of estrus (day 17) as the suppressive effects of progesterone are removed and probably as a consequence of the positive stimulatory action of estrogen on gonadotropin-releasing hormone pulse frequency. Concentrations of luteinizing hormone peak about 2 days after ovulation and then decline slowly. Because of its prolonged half-life, luteinizing hormone reaches baseline concentrations at about day 5 or day 6 after ovulation.³

Ovarian function

Estrogen induces the major structural changes throughout the genital tract that are associated with estrus, including:

• Softening and relaxation of the cervix.

• Increased fluidity and volume of uterine and vaginal secretions.

• Uterine edema.

• Growth of the primary follicle.

It also acts on the brain, stimulating behavioral changes of estrus and having both positive and negative feedback on gonadotropin-releasing hormone (see Fig. 5.9). Shortly before the luteinizing hormone surge, plasma and urinary estrogens rise, reaching a peak 24–36 hours before ovulation. Estrogens are secreted mainly as estrone and 17β-estradiol. In the absence of concurrent high progesterone concentrations, high concentrations of estrogen cause estrous behavior.

Progesterone is secreted by the corpus luteum during days 1–17 after ovulation. Progesterone causes contraction of the cervix and increased viscosity of vaginal secretions. It stimulates proliferative changes in the uterine luminal epithelium and endometrial glands and acts on the brain to induce the rejection-type behavioral response shown by mares in diestrus. Plasma progesterone concentrations are low (less than 1 ng/ml) throughout estrus but rise sharply after ovulation, reaching values of 1.5–2.5 ng/ml by 24 hours and 2–5 ng/ml by 48 hours post ovulation (day 0). Peak concentrations of 8–20 ng/ml are attained by days 5–8 after ovulation, and these values remain high until luteolysis commences at days 14–15 after ovulation (Fig. 5.10).

• The rapid rise in plasma progesterone concentrations after ovulation makes progesterone assay a useful clinical tool for confirming ovulation.

Oxytocin is released from the posterior pituitary by the estrous mare in response to sexual arousal, particularly if the arousal is a stallion call. During teasing, breeding, and artificial insemination, peaks in oxytocin levels are detected in pituitary venous blood. Exogenous oxytocin, now frequently administered as a postbreeding treatment, may stimulate luteinizing hormone secretion slightly, possibly advancing ovulation.

• Oxytocin also stimulates oviductal and uterine contractions, facilitating the union of ovum and sperm in the oviduct, and assisting uterine clearance after breeding.

Graafian (mature) follicles secrete inhibin during their maturation. This hormone exerts a direct, negative feedback effect on follicle-stimulating hormone release, presumably by altering the sensitivity of follicle-stimulating hormone gonadotropins in the pituitary to stimulation by gonadotropin-releasing hormone.

Early estrus

The mid-cycle release of follicle-stimulating hormone (Fig. 5.11) stimulates follicular development. Around days 16–18 after ovulation, near the beginning of estrus, a few follicles (greater than 25 mm in diameter) are present, and one follicle is associated with the luteinizing hormone surge beginning on day 16 or 17 after ovulation, when levels of follicle-stimulating hormone are lowest. Resulting estrogen production from these growing follicles initiates the physical and behavioral changes of early estrus.

Fig. 5.11 Cyclical variations in circulating hormones during the normal estrous cycle of the mare.

Late estrus

As the dominant follicle approaches maturation, a large preovulatory surge of estrogens produces a positive feedback on the pituitary, continuing the rise in luteinizing hormone concentrations. In contrast to females of other species, mares have a prolonged surge in luteinizing hormone levels that often lasts for 7–8 days and usually peaks after ovulation (Fig. 5.11). Because it is common for multiple follicles to develop during estrus, this long exposure to luteinizing hormone may account for the large number of multiple ovulations that occurs.

Ovulation

In its embryonic development, the cortex of the equine ovary folds at its hilus and becomes surrounded by medullary tissue. Mechanically, this only allows ovulation through the hilar area or ovulation fossa. Follicles are palpated on the general ovarian surface, but the follicle collapses at ovulation and forces the ovum through its tract to the ovulation fossa. Follicles usually increase rapidly in size (about 5 mm in diameter per day) before ovulation. Mares weighing 400–550 kg often ovulate from follicles 45–65 mm in diameter, whereas smaller mares weighing 225–350 kg may ovulate from follicles 35–45 mm in diameter.¹⁷

At ovulation, as detected by rectal palpation, there may be a perceptible softening of the follicle before rupture, but this is not a consistent finding. Mares frequently exhibit signs of mild pain at the site of a recently collapsed follicle, probably associated with the sudden change in tension of the visceral peritoneum that surrounds all but the ovulation fossa of the ovary.

The time of ovulation in relationship to estrus varies. It is virtually impossible to predict ovulation time based solely on the duration of estrus. One report indicated that only 46% of the mares studied ovulated on the day before the end of estrus and nearly 12% ovulated 3 days before the end of heat. Very few mares (1.4%) ovulated after sexual receptivity had ceased.

Ovulation usually occurs during evening or night hours, with approximately 75% of mares ovulating between 4:00 p.m. and 8:00 a.m. The incidence of multiple ovulation reportedly varies from 14.5% to 42.8%.¹⁸

Endocrine changes after ovulation are quite rapid in most mares, and physical and behavioral changes follow very shortly (Table 5.2; see also Fig. 5.7). Some luteinization probably occurs before ovulation; however, estrogen concentrations drop drastically when the follicle collapses and the corpus luteum develops quickly. The resulting reversal in circulating estrogen:progesterone ratio causes the rapid cessation of estrous behavior. At this point, progesterone rather than estrogen dominates behavior and the physical character of the tubular genitalia.

Table 5.2

Endocrine and physical characteristics of the estrous cycle of the mare (FSH, follicle-stimulating hormone; LH, luteinizing hormone)

Diestrus

The corpus hemorrhagicum is palpable per rectum during the first 2–3 days after ovulation as a soft, spongy structure on the surface of the ovary. As it matures into a corpus luteum and the blood within the corpus hemorrhagicum clots, the consistency changes to that of a rubbery, firm structure that is difficult to distinguish from a firm follicle (Fig. 5.12). The ultrasonographic appearance of the corpus luteum is characteristic and is easily identified throughout its lifespan.

Fig. 5.12 Normal ovary, showing mature corpus luteum (brownish/yellow tissue; arrowed), smaller corpus luteum and atretic follicles. Note the thickened walls of atretic follicles.

The corpus luteum is considered to be mature 5 days after ovulation. Until that time, the corpus luteum is refractory to luteolysis by prostaglandin. The mature corpus luteum induces high blood concentrations of progesterone (8–10 ng/ml) until about days 14–15 of the estrous cycle. At this time, the corpus luteum undergoes luteolysis in response to PGF₂_α, which is released from the endometrium. PGF₂_α enters the systemic circulation and reaches the corpus luteum via arteriovenous transfer and/or in the lymphatics, where it precipitates luteolysis and so instigates the next estrous cycle.

RECOGNITION OF ESTRUS IN THE MARE

The correct and early recognition of estrous behavior in mares is an important part of stud farm management. Failure to detect estrus and the consequent failure to breed mares is a common (and largely unnecessary) cause of poor conception rates.

• Early recognition results in better synchronization of mating with ovulation and reduces the number of rectal examinations that may be required.

Mares that fail to show overt estrous behavior may be more difficult to assess. Where visual inspection and obvious behavioral changes do not detect estrus, teasing (testing with a teaser pony, testosterone-implanted gelding or stallion) is standard practice in stud farm management. Although this procedure may seem to be tedious and time-consuming, it is clearly vital for the success of the breeding program. The best stud farms with the best results are expert at the process of teasing and take particular care to try and detect early signs of estrus in the mares, often when the behavioral and physiological changes are subtle.

Detailed breeding records may be very helpful in predicting when the mare is likely to come into estrus, although there may be variations in cycle length during the season and variations may also arise from early embryo loss, etc. (see p. 163).

Repeated examinations of the reproductive tract are also helpful in determining when the mare is in estrus, especially if there is no stallion available or she does not exhibit estrous behavior.

By far the most efficient method for estrus detection is to tease each mare individually with a stallion, but mares can also be teased by teasers, or on occasion by geldings (see p. 153). The interrelationship between a mare and stallion during estrus is probably more dependent on smell and taste rather than on sight. However, teasing is usually restricted to one or two 5–10-minute periods each day and this is clearly an artificial circumstance. While most mares will either show signs of estrus or diestrus (rejection behavior and absence of clinical evidence of estrus), some mares may not respond normally to this narrow time scale and often appear to be in diestrus throughout the cycle. The extent of estrus ‘showing’ is variable. Some respond quickly to contact with the stallion (often even on initial approach), but other mares may take considerable time to settle to the behavior pattern.

Managerial factors may influence the extent to which estrus is shown by behavioral signs alone. Management that involves separation of the mares from the stallions for the greater part of the time means that teasing (testing) must be performed more effectively.

• Maiden mares may also be difficult, especially those that are nervous and excitable; they may be reluctant to show estrus and may then not respond well even when teased individually.

• Mares do not show any overt sexual behavior (e.g. mounting each other) when grouped with other mares and overt changes in the physical characteristics (mucous secretions) of the vulva are not usually obvious.

• Mares with a high androgen concentration (resulting from administration of male hormones or anabolic steroids or certain ovarian tumors) may show masculine behavior rather than estrus.¹⁹

• Estrus signs may be much stronger in the presence of an active stallion. Estrus in paddock or free-bred mares will be detected quickly by the attending stallion(s).

• Mares with foals at foot may not show strong signs of estrus, either if the foal is separated from her at the time of teasing or if the foal is physically beside the mare during teasing. This is caused either by separation anxiety or by protectiveness, respectively.

Estrous behavior is defined as the pattern that results in acceptance of the approach of, and mating by, a stallion. Usually a mare is considered to have been in estrus if she shows 4–6 days of estrous behavior followed by a period of 10–14 days of diestrus during which she should show no receptivity to the stallion and no overt clinical signs of estrus ²⁰ (see below). Individual mares may habitually show strong or weak estrous signs. In the latter cases, supportive testing (see alternative methods below), dates and records can be helpful.

Behavioral changes typical of estrus include:

• Docility (although behavior can be unpredictable).

• Ears relaxed either forward or in neutral position.

• Vocalization in the form of squealing often occurs with laid back ears when the mare is first approached by the stallion.

• Tail raising and swishing with adoption of a stance similar to urination but maintained for longer periods and without evidence of straining. The tail is usually held slightly to one side.

• Fence pushing.

• Lengthening and eversion of the vulvar lips with clitoral eversion (‘winking’).

• Squatting and frequent squirts of urine (usually with a characteristic odor and yellowish cloudy appearance).

Alternative methods of estrus detection

These are useful if either a teaser is not available (often for cost or management reasons) or the stallion cannot be used to tease the mare (usually for safety reasons and because some stallions become frustrated and develop behavioral problems such as self-mutilation or indifference to breeding). In any case, a single method should not be relied upon because there are variations from mare to mare.

Vaginal examination

Vaginal examination of estrous mares often identifies a thin mucoid secretion. Attempts to measure the electrical conductivity of vaginal mucus have not been rewarding. Vaginal cytology has also not proven to be an effective mechanism for estrus detection.

Vaginoscopic examination (Fig. 5.13) can be used to detect estrus (and diestrus) with some accuracy but it is a semi-invasive method and can cause problems with vaginal inflammation.

Fig. 5.13 Vaginoscope in use.

The normal state of the cervix changes during estrus (and during pregnancy).²¹ During estrus the cervix shows hyperemia (a red or dark pink color). The mucosa is edematous (moist-looking and thickened) and the cervix itself is relaxed and lies on the floor of the vagina. The mucus consistency correlates closely with the rise in blood estrogen and is recognized as an increased shininess of the mucosal surfaces. Introduction of the speculum is relatively easy due to increased mucous secretion during estrus but commonly causes a significant increase in blood vessel diameter and increased redness of the mucosal surfaces.

Note: