hCG and Ovulation

Managing a mare's estrous cycle is an integral part of breeding management, and human chorionic gonadotropin (hCG) is one hormone option for doing just that. Patrick McCue, DVM, PhD, Dipl. ACT, associate professor of equine science at Colorado State University, discussed the use of hCG to manage ovulation.

"Most of us have used hCG throughout our practice lifetimes," he began. "It's used to induce a timed ovulation in in-heat mares. We expect that 75-85% of mares will ovulate within 48 hours after hCG administration, and the average interval to ovulation is about 36 hours."

He also briefly discussed the history of another ovulation induction agent--Ovuplant (deslorelin acetate, an analog of gonadotropin-releasing hormone or GnRH). The difference between the two is that hCG mimics the activity of luteinizing hormone (LH), which hastens the maturation and induces ovulation of the dominant follicle in a mare.

In comparison, deslorelin stimulates the pituitary gland to secrete endogenous LH, which will in turn cause follicle maturation and ovulation. Ovuplant was an implant that was designed to be inserted under the skin. When it was first commercially available, it was noted that some mares induced to ovulate with Ovuplant had a delay in their return to estrus. Studies showed that when the implant was removed after ovulation was detected, the incidence of failure to return to estrus was reduced. However, he said that by the end of 2003 the implant was no longer available in the United States. Compounded deslorelin has been used in the equine breeding industry in the past few years, but future availability of the compounded product is uncertain.

Consequently, hCG may once again be the only approved drug available for induction of ovulation in mares. McCue indicated that "It is controversial as to whether or not repeated use of hCG within a breeding season is associated with a loss of efficacy at inducing a timed ovulation. Repeated administration of hCG has been documented to result in formation of anti-hCG antibodies. Adverse effects, if any, are presumed to be due to the presence of these antibodies." Some studies have shown a reduced efficacy with multiple use, while others have failed to demonstrate an adverse effect.

The goal of the CSU study he presented was to evaluate data from a large number of mares given hCG to determine the effects of repeated administration during a given breeding season on efficacy, and to determine if a relationship exists between mare age and hCG effectiveness in inducing a timed ovulation. Their study found that 78% of all mares ovulated within 48 hours of hCG administration on the first cycle of the year, and about 4.4% failed to ovulate in response to hCG.

When hCG was administered multiple times within a single breeding season, the percentage of mares ovulating within 24 hours after administering hCG increased and the percentage of mares ovulating between 24 and 48 hours decreased, McCue reported. In addition, the percentage of mares ovulating three to four days after hCG administration increased in later cycles during the breeding season. It was noted that a previous study had reported a seasonal effect in which an increase in the frequency of ovulation less than 24 hours after hCG administration occurred later in the spring.   

The CSU study also reported a decrease in the efficacy of hCG in inducing a timed ovulation with increased mare age. He noted that a similar decrease in efficacy with age was reported for Ovuplant.

McCue noted that it was common practice to use hCG for the first cycle or two during a breeding season and then switch to an alternative ovulation-inducing agent, such as deslorelin (when available), for subsequent cycles. But "It is unknown whether any form of deslorelin will be available during the 2005 breeding season," he concluded. "If hCG is the only agent to be used to induce ovulation in mares bred during multiple estrous cycles within a single breeding season, such as donor mares in an embryo transfer program, the ability to accurately predict the time of ovulation may be reduced later in the year."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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