The Effect of Postinsemination Endometritis on Fertility of Frozen Stallion Semen

In mares, a physiological endometritis ensues following the deposition of semen into the uterus. This response is thought to be due to the antigenic nature of spermatozoa, specifically the activation of complement, and may be modulated by the presence of seminal plasma component of the semen. The removal of seminal plasma during cryopreservation of spermatozoa may account for the longer duration of the inflammatory reaction that often accompanies insemination with frozen-thawed semen. This reaction, following insemination with fresh semen, appears to be greatest at 12 hr after insemination, gradually subsiding in 24–36 hr. In vitro studies indicate that sperm motility is most significantly depressed at 12 hr following insemination as well. Some of the highest reported pregnancy rates using frozen semen have been the result of a breeding protocol where mares are bred both preand postovulation with 6–10 hr between inseminations. Since the cost of a single breeding dose of frozen semen is often quite expensive, researchers have questioned if the second dose, inseminated in such a proximity to the first dose, is truly necessary for acceptable pregnancy rates, or the sperm are simply phagocytized by the incipient endometritis, never to reach the oviduct. The present experiment, using heterologous spermatozoa, was designed to determine if the fertility of frozen-thawed semen is lost when inseminated within 4–10 hr of a previous dose of frozen-thawed semen.