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Reproductive System of the Mare

Reproductive System of the Stallion

Short-Cycling Mares | Super-Ovulating Mares

Collecting Semen

Training to AV & Phantom | Semen Analysis

Freezing & Storing Semen | Shipping Semen

Breeding With Fresh or Frozen Semen

Breeding With Shipped Semen | Breeding on Foal Heat

Breeding Older Mares | Embryo Transfers

  Reproductive System of the Mare



Reproductive System of the Stallion




Short-Cycling Mares

At Veterinary Services, we routinely short-cycle or manipulate the reproductive cycle in mares to cause them to come into estrus (heat) at a designated time rather than when nature would dictate. The injection of a particular hormone at the right time can cut a mare's normal estrous cycle in half (or more).

The prime tools involved in manipulating reproductive cycles are two naturally occurring hormones--prostaglandin and progesterone--or their synthetic counterparts.

Short-cycling can be achieved by administering prostaglandin alone. Timing is the key. If prostaglandin is administered when the mare is in a state of diestrus--out of heat--it has the same effect as the prostaglandin produced by the endometrium of the uterus when there is no pregnancy. The corpus luteum is immediately rendered incapable of producing progesterone. When this occurs, other hormonal levels instantly rise and bring the mare into estrus within a few days.



SuperOvulating Mares

Fertility in the mare begins with ovulation. The mare naturally ovulates one ovum (egg) per 21-day cycle. In a typical breeding season, February through July, this presents six to eight opportunities for ova to be fertilized. Compared to the mare's spontaneous (unassisted) ovulation, superovulation maximizes the fertilization opportunities by inducing the development and ovulation of a greater number of follicles. This method can optimize fertility--ova mature in follicles, so a greater number of follicles means more oocytes and an increased likelihood of conceiving more foals.


Superovulation maximizes the fertilization opportunities

by inducing the development and ovulation

of a greater number of follicles.


With a higher probability of establishing a pregnancy, superovulation might result in the fertilization of multiple oocytes. This gives us a possible opportunity to collect more than one embryo for implanting into donor mares via embryo transfer.

Superovulation stimulates the entire follicular wave. Those follicles that might have degenerated--or not have been selected to ovulate--will go ahead and ovulate in addition to the dominant follicle.

Equine pituitary extract (EPE) has succeeded in producing multiple ovulations. This preparation consists of gonadotropins from the horse's pituitary gland. At CSU, gonadotropins are extracted and processed to yield the EPE compound. McCue defined the resulting EPE as containing "approximately 6-10% LH (luteinizing hormone) and 2-4% FSH."

He described the use of EPE to superovulate mares at CSU.

"The exact day of ovulation is determined and the mares are then short-cycled with prostaglandin (PGF2alpha) five days later and administered 30 mg. of EPE as an intramuscular injection once daily." Prostaglandin destroys the corpus luteum, and it starts the estrous cycle just prior to the beginning of the next follicular wave. EPE treatment lasts approximately six to eight days.

"When multiple follicles greater than 35 mm in diameter are detected, the mares receive 2500 IU human chorionic gonadotropin (hCG) intravenously to promote synchronous ovulations," said McCue. A mare is bred prior to ovulation and also when she ovulates, immediately after researchers detect ovulation.

Recent research has shown that EPE provides better multiple ovulation results than other hormone treatments. For example, researchers have studied the effects of gonadotropin-releasing hormone (GnRH), which helped improve ovulation rates through multiple ovulations in seasonally non-cycling mares. However, this hormone showed less-promising results on cycling mares.

At CSU, the results of superovulating 55 mares in the embryo transfer program in the 1994 and 1995 breeding seasons were an overall 2.4 ovulations per cycle, with a range from one to five. Of mares treated with EPE, 74% had multiple ovulations. Researchers elsewhere had reported as many as 10 ovulations in one mare due to EPE treatment.

Mares were bred with either shipped cooled semen or frozen-thawed semen, and pregnancies monitored with ultrasound. Results were double the number of embryos collected per donor (from the superovulated mares as compared to non-treated mares).

Other studies at CSU have investigated whether superovulating normal mares can lead to increased pregnancy rates in mares bred to a subfertile stallion. McCue noted that mares ovulating multiple follicles had a 67% pregnancy rate, compared with 33% for the mares with a single follicle. This result was for mares inseminated with a limited dose of frozen semen.




Copyright 2005

Veterinary Services




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