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Manipulation of oestrus in the mare
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Breeding management of mares has evolved into an art that involves incorporating different therapies and breeding modalities. Depending on the method of breeding chosen (ie, live mating vs artificial insemination), approaches vary. However, fundamental principles remain the same and include:
- Ensuring good fertility in the mare and stallion
- Use of good breeding management techniques
- Breeding as close to ovulation as possible
- Optimizing uterine health in the peri-ovulatory period
Several tools can be employed to aid the veterinarian while performing breeding management practices. Useful tools include, but are not limited to, assessing behavioral estrus when a mare is exposed to a stallion, evaluation of the reproductive tract using transrectal palpation and ultrasonography, pharmacologic induction of estrus and ovulation, and post-breeding therapy.
Assessment of behavioral estrus
Exposing a mare to an intact stallion is one of the simplest and most reliable tools available for determining if a mare is in estrus. With the changing demographic of horse owners, many mares are housed in training barns or private properties that do not have intact stallions present. While some mares may show signs of estrus to geldings or other mares, this behavioral response is not nearly as reliable as exposure to a stallion.
The mare in estrus classically shows a positive response to a stallion by approaching him with her ears forward and interest. In addition to making nose to nose contact, the mare will position her hind end to the stallion, tail flag, passively posture, urinate and expose her clitoris (“winking”). In some cases, the mare shows reluctance to leave the stallion. Timid or maiden mares often do not show signs of behavioral estrus as readily, and they may require additional time/training before feeling comfortable enough to be behaviorally “honest.” In all cases, a mare should be exposed to a stallion daily when she is in estrus to best determine her pattern of behavior.
Diestral (or pregnant) mares show little interest in stallions and may be aggressive. Typical behavioral signs expressed by a mare that is not in estrus include ears pinned, tail clamped, striking, kicking, squealing and reluctance to position her hind end toward the stallion.
Taking control of the estrus cycle
Commonly, the mare owner does not have access to a stallion for teasing, and often does not know what stage of the cycle the mare is in when first entering a breeding program. Using transrectal palpation and ultrasound, the mare’s stage of the estrous cycle can usually be determined. (See Table 1) While it is ideal for the veterinarian to begin examining a mare for breeding when she first enters estrus, often the mare is in diestrus at the first examination. The options for managing the diestral mare are to 1. Wait until the mare returns to estrus naturally (at the risk of missing estrus), or 2. Induce estrus (through luteolysis) by administration of prostaglandin F2α.
“Short cycling” or induction of luteolysis to bring the mare into heat
In the presence of a mature corpus luteum (i.e. in diestrus), prostaglandin F2α (PGF) may be administered to cause luteolysis and allow the mare to come into estrus. Because administration of PGF shortens the duration of diestrus, it is often called “short cycling” the mare. Prostaglandin products available in the United States include dinoprost tromethamine (Lutalyse®, 5-10 mg, IM, Zoetis Animal Health) and cloprostenol (Estrumate®, 250 mcg, IM, Schering Plough). Both products are widely used to induce luteolysis. Advantages of dinoprost tromethamine are low cost and rapid effect. Disadvantages are side effects including colic and profuse sweating. Cloprostenol induces fewer side effects and has a longer half-life after administration but is more expensive than dinoprost.
Prostaglandin F2α is effective for inducing luteolysis between days 5 and 15 of the estrus cycle (ovulation = day 0). The average mare returns to estrus 5 – 7 days after PGF administration and ovulates 7-10 days after prostaglandin treatment. Examining the mare’s reproductive tract prior to PGF administration is essential for predicting a return to estrus. Follicle size at the time of PGF administration dictates the interval to estrus and ovulation. Mares having larger follicles (30-40 mm) when PGF is administered will return to estrus more quickly, sometimes within a few days. Therefore, it is prudent to begin reproductive examinations as early as 2 days after PGF if the mare has medium to large follicles on her ovaries. In contrast, mares having small to medium follicles (5 – 20 mm) on their ovaries at the time of PGF administration often do not return to estrus for at least 3 – 5 days. In all cases, examination of the mare’s reproductive tract earlier, and more frequently, is the best solution for identifying when she returns to estrus.
Predicting ovulation for breeding
Once the mare has entered estrus (follicles ≥ 30 mm, softening of the tubular tract, ultrasonographic presence of uterine edema), daily reproductive examinations aid in predicting the best time to breed the mare. Equine follicles generally grow a rate of 3 – 5 mm per day and attain an average pre-ovulatory size of 40 mm. Follicular shape and texture have been advocated as reliable means of predicting ovulation. Much of the early work characterizing these changes involved very frequent examinations of the reproductive tract so that subtle changes in the tract were readily identified. Through a complex process of tissue remodeling, follicular architecture changes in the days prior to ovulation which causes the shape and texture of the follicle to change. Early in estrus, increasing diameter is the most notable alteration to pre-ovulatory follicles. As the follicle nears ovulation, the shape may change from spherical to oval. Additionally, a point may form on the follicle as tissue remodels in the region of the ovulation fossa, which facilitates rupture and release of the oocyte. While some will use the presence of a point or cone as a means of predicting imminent ovulation, it has been this author’s experience that this change occurs in few hours prior to ovulation. With once daily examination of the reproductive tract, identifying a follicular point is not consistent and cannot be relied on to predict ovulation. However, as the follicle remodels, the tension on the follicular wall will decrease causing the follicle to become palpably softer. While also subjective, softening of the follicle is a more general architectural change that can be detected with daily examinations of the reproductive tract.
All changes to the follicle must be considered in conjunction with concurrent changes to uterus and cervix. With increasing concentrations of estrogens, the uterus and cervix will become palpably softer in texture and more edematous when examined using transrectal ultrasound. These factors are critical to successful prediction of ovulation, and importantly, to accurate administration of an ovulatory agent.
Induction of ovulation
The process of ovulation can be initiated through pharmacologic means. Human chorionic gonadotropin (hCG) and gonadotropin releasing hormone analogs (GnRH; deslorelin acetate, histrelin) are commonly used agents in mares. Human chorionic gonadotropin exerts LH-like activity in the mare through binding to LH receptors in granulosa and theca cells in follicles. When 1500 to 5000 IU hCG are administered to a mare with a follicle > 35 mm in diameter, in conjunction with additional signs of estrus (estrus edema in the uterine wall, cervical relaxation), ovulation occurs approximately 36 hours later in 80% of mares. More recently, workers have demonstrated that lower doses of hCG administration (750 IU vs 1500 IU) are as effective for inducing ovulation in mares as higher doses.
Deslorelin acetate, first marketed in an implant formulation, is a GnRH analog that is used in mares to induce ovulation. When administered to estrus mares having a follicle of > 30 mm, ovulation can be expected approximately 42 (40 – 48) hours later. Deslorelin acetate is now produced in an injectable formulation with similar properties for inducing ovulation as the implant (SucroMate, Thorne Bioscience, Louisville, KY) Histrelin, also a GnRH agonist (500 µg, IM) has been shown to have similar efficacy for inducing ovulation as deslorelin acetate when administered to mares with a follicle > 30 mm in diameter.
Insemination of the mare
Timing of insemination relies on the method of breeding used. The goal for mares bred by live cover or artificial insemination using fresh or cooled semen is to have viable sperm in the oviduct of the mare prior to ovulation. Oocytes remain viable for approximately 24 hours after ovulation, however, a higher incidence of early embryonic death occurs in mares bred 12 or more hours after ovulation. Equine sperm, on the other hand, can remain viable in the reproductive tract of the mare for several days. Sperm viability after ejaculation is highly stallion dependent and should be factored into breeding management decisions.
Management of mares for insemination using frozen-thawed semen requires greater attention to detail due to the limited viability of frozen-thawed sperm. Timed breeding programs using 2 doses of frozen-thawed semen (before and after ovulation) provide good pregnancy rates and a more convenient insemination schedule vs one time insemination immediately after ovulation. Method of insemination for frozen-thawed semen is often dictated by the amount of semen that is available for each breeding cycle.
Post-breeding Evaluation and Treatment
Documenting ovulation with ultrasonographic examination of the reproductive tract is important for determining when to perform a 14-16 day pregnancy examination and to rule-out the presence of twin embryos. At the time of documenting ovulation, the veterinarian can also assess the uterus for persistent uterine edema or intraluminal fluid. Possible treatments used to aid in resolution of uterine inflammation include drugs to induce uterine smooth muscle contractions (e.g. oxytocin or cloprostenol), uterine lavage and antimicrobial therapies. Treatments vary depending on the cause, and degree, of intrauterine fluid detected. The most commonly used therapies are ecbolic agents such as oxytocin (10-20 units, IV or IM, q4-8h) and cloprostenol (250 mcg, IM, q6-8h). It is important to note that cloprostenol should not be administered after ovulation as it can interfere with progesterone production of the developing corpus luteum.
Summary
The equine veterinarian has many options for breeding mares. Breeding methods have become more sophisticated with the addition of tools that aid in detection and management of estrus in mares. In present day practice, it is common for the veterinarian to “take charge” of the estrous cycle to maximize success using a variety of breeding modalities.
Keywords: mare, breeding management, ovulation induction, prostaglandin, hCG, deslorelin acetate
Stage of the cycle | Endocrine Changes | Physical Changes | Behavioral Response | ||
Pituitary | Gonadal | Ovary | Tubular Tract | ||
Early Estrus | FSH rise | Estrogen rise | Follicle (s) develop; firm texture | Slight uterine edema; cervix relaxing (pale pink); moderate tone | Beginning receptivity; interest but mixed signals |
Late Estrus | LH surge | Estrogen peak | Follicle (s) mature; soft and fluctuant | Maximum uterine edema and cervical relaxation (pink);mild tone | Strong receptivity, tail up, ears forward, wink, posture |
Ovulation | LH peaks after ovulation | Estrogen declines, progesterone rises | Follicle collapses (crater); corpus hemorrhagicum (CH) forms | Uterine edema declines; relaxed cervix (dark pink); flaccid tone | Strong receptivity |
Early Diestrus | LH decline | Progesterone rise | Late CH to early corpus luteum; rarely palpable | Uterine edema gone; cervix closing (pink), mild/moderate tone | Rejects stallion; ears back, squeal, tail clamp, kick |
Mid-diestrus | FSH rise (follicular recruitment) | Progesterone peak | Follicular activity; CL visible but not palpable | Toned tract; no uterine edema; closed, tubular cervix (pale pink) | Rejects stallion; ears back, squeal, tail clamp, kick |
Late Diestrus |
| Luteolysis; progesterone falls | Initial follicular development | Early softening of uterine tone and cervix (pale pink) | Rejects stallion; ears back, squeal, tail clamp, kick |
Table 1: Physiologic and behavior changes during the estrous cycle of t
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[1] Douglas RH, Ginther OJ. Effect of prostaglandin F2alpha on length of diestrus in mares. Prostaglandins 1972;2:265-268.
[2] Ginther OJ. Reproductive Biology of the Mare: Basic and Applied Aspects. 2 ed. Cross Plains, WI: Equiservices, 1992.
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