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