AAEP Convention 2005: Optimizing Pregnancy Rates for Frozen-Thawed Semen

A major disadvantage associated with frozen-thawed semen is the fertility of the semen, which can be considerably less than fresh semen. This can be problematic when deciding how many progressively motile sperm (PMS) are needed per breeding to produce optimal pregnancy rates in mares. Elizabeth Metcalf, MS, DVM, Dipl. ACT, of Honahlee PC equine clinic in Oregon, presented her findings regarding varied concentrations of motile sperm in frozen semen doses at the 51st Annual American Association of Equine Practitioners Convention, held in Seattle, Wash., Dec. 3-7, 2005.

"Motility does not always reflect fertility," Metcalf said. "When we examine the motility of sperm, we have to realize that many other factors affect the pregnancy rates of fertile mares."

Her study included 90 mares bred over 312 cycles with frozen-thawed semen from 46 stallions, which represented a large number of breeds (Quarter Horse, Thoroughbred, Paint, Morgan, Arab, etc.). Mares were bred 36 to 44 hours after administration of deslorelin (hCG, a heat-cycle inducing agent). The mares ideally would ovulate between breedings.

"The total amount of sperm inseminated into each mare, per cycle, ranged from less than 100 to more than 800 million progressively motile sperm," Metcalf explained. Pregnancy rates in the study increased in correlation to the number of PMS used, peaking at 600-800 PMS (88.2%), and then decreasing at higher concentrations.

"More is not better here," Metcalf explained. "More than 800 million progressively motile sperm yielded significantly lower pregnancy rates than lower doses." She explained that this could be due to a less fertile population of mares or an unresolved inflammatory response secondary to presence of large numbers of sperm.

"Insemination pre-ovulation and post-ovulation yielded higher pregnancy rates than a single post-ovulation dose," Metcalf said.

"The study suggests the number of sperm we should be using per dose--between 600-800 million progressively motile sperm split into two doses," Metcalf said. "It also suggests a protocol for breeding mares at 36 and 44 hours" after giving an ovulation-inducing medication.

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for TheHorse.com .

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