CHAPTER 3Estrous Synchronization

As a prelude to this chapter, it is necessary to make a few comments about estrous synchronization as it pertains to the mare and how this varies from estrous synchronization in the cow. Practitioners who are familiar with estrous synchronization in cattle and the number of different synchronization programs available must realize that estrous synchronization in the mare is much more relative and does not yield as reliable results as estrous synchronization in cattle. In the bovine industry a large number of cows can be synchronized to reliably display estrus and ovulate within a narrow window of time, and there are several programs described to achieve the desired results. Unfortunately, the mare’s reproductive cycle does not lend itself to such a tight synchronization among a group of mares, and currently there is only one protocol that consistently provides reliable results. The length of estrus in the mare is 5 to 7 days, and the time of ovulation relative to the end of estrus varies with ovulation occurring on the last 2 days of estrus in 69% of mares and after the end of estrus in 14% of mares.¹ These factors make the development of a synchronization program that allows for a single breeding at a predetermined time difficult.

In equine reproduction the most common use of estrous synchronization is in embryo transfer programs. If a large recipient herd providing mares that have ovulated in synchrony with the donor is not available, then a small group of recipient mares must be synchronized with the donor mare. To be suitable recipients, these mares must ovulate from 1 day before to 3 days after the date of ovulation of the donor mare. There is even one report in the literature that recipient mares that ovulated up to 5 days after the donor have been used successfully in a large embryo transfer program.²

A second less common use of estrous synchronization in the mare is to synchronize a group of mares for breeding due to limited availability of a stallion, for example due to a show schedule.

In addition to synchronization of estrus in a group of mares, pharmacologic agents can be used to induce ovulation to allow for the synchronization of ovulation. When breeding with both frozen and cooled shipped semen it is often desirable to use only one dose of semen, and therefore it is essential that the ovulation be synchronized within 36 to 48 hours of breeding. This is also important where natural service of mares is performed and the stallion has a full book of mares, thereby limiting the number of times a mare can be bred during a cycle.

There are four basic methods used to achieve estrous synchronization in the mare: termination of the luteal phase, lengthening of the luteal phase, induction of ovulation, and inhibition of the follicular phase. The pharmacologic agents most commonly used to accomplish these methods are prostaglandins, progestins, human chorionic gonadotropin (hCG) (Chorulon, Intervet Inc., Millsboro, Del.) or deslorelin acetate (Ovuplant, Peptech Limited, NSW, Australia), and estradiol-17β. The most reliable and successful estrous synchronization programs are achieved when a combination of these methods and pharmacologic agents are used.

Prostaglandin F_2α(PGF_2α) is used to terminate the luteal phase by causing lysis of the corpus luteum (CL), thereby allowing the return to estrus. The most common prostaglandins used are dinoprost (Lutalyse, Pfizer, New York) and cloprostenol (Estrumate, Schering-Plough Animal Health Corporation, Union, N.J.). Dinoprost is a naturally occurring PGF_2α, and cloprostenol is a synthetic prostaglandin analogue. For prostaglandins to effectively terminate the luteal phase a mature CL must be present that is responsive to prostaglandin. The equine CL exhibits incomplete sensitivity to PGF_2αuntil approximately day 5 postovulation in most mares.³ The standard recommended dose of dinoprost is 9 μg/kg (5 to 10 mg per 1000 lb/454 kg horse). The recommended dose of cloprostenol is 250 μg per 1000 lb/454 kg horse (0.55 μg/kg). More recent research has shown that much lower doses of prostaglandin are just as effective at lysing the CL with fewer negative side effects. Douglas and Ginther demonstrated that doses of prostaglandin as low as 1.25 mg were as effective at inducing luteolysis as more standard doses.⁴ Nie et al showed that a microdose of cloprostenol (25 μg) was just as effective at lysing the equine CL as the larger standard dose.⁵

On average, mares return to estrus 5 to 7 days after administration of prostaglandin in the presence of a mature CL, and ovulation occurs 9 to 11 days after administration. However, because the follicular phase plays a greater role in controlling the total length of the estrous cycle in the mare, the length of time from administration of prostaglandin to onset of estrus and ovulation can be quite variable depending on the size and status of follicles present on the ovaries at the time of administration. The time of ovulation after PGF_2α-induced estrus may vary as much as 2 to 15 days post treatment.⁶ If prostaglandin is administered when there is a large follicle present, the mare may return to estrus and ovulate in 2 to 3 days. Some of these mares never develop overt signs of standing heat, endometrial folds, or relaxation of the cervix before ovulation. If a large follicle destined for atresia is present on the ovary at the time of prostaglandin administration, the follicle may not ovulate and instead regress as a new wave of follicles develops.⁷

In cattle the administration of two doses of prostaglandin 11 to 12 days apart results in approximately 80% exhibiting estrus within 2 to 5 days after the second injection. Although the administration of two doses of prostaglandin 14 days apart has been described in the literature as a protocol for estrous synchronization in mares, it is not a very reliable method of ensuring that a recipient mare will be synchronized with a donor.⁶ Research has shown that after two injections of prostaglandin, on average ovulation occurs between 7 and 10 days after the second injection,^6,8,9 with ovulation potentially occurring anywhere between 0 and 17 days.^8,9 In a review of the literature it was stated that when this protocol was administered, one would need to treat at least 10 recipient mares to have an 80% chance that at least 1 recipient would ovulate within 24 hours of the donor.¹⁰ Obviously, this is not ideal, especially when in most small embryo transfer programs there are only a few mares available to be used as recipients. Due to the large variation in time of ovulation from administration of prostaglandin, it is not very useful when used alone in the synchronization of estrous cycles in mares. However, if examination via transrectal ultrasonography is performed first to eliminate those mares that will not respond appropriately to the administration of prostaglandin, tightening of date of ovulation can be achieved with this protocol.

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