Maximizing Pregnancy Rates With Shipped Semen

Well, there’s no avoiding it—shipped semen is no longer the ‘wave of the future.’ It has rapidly emerged as an integral part of every general equine practice and breeding facility. Therefore, we must pay rapt attention to all means of enhancing the fertility of stallion semen being shipped nationwide. There is, however, tremendous variability in the fertility of semen shipped throughout the country and subsequent pregnancy rates.

Pregnancy rates of mares bred with cooled shipped semen are dependent on three variables: 1.) the fertility of the mare. 2.) the fertility of the semen with which they are inseminated. and 3.) the expertise of the veterinarian coordinating the insemination.

Unfortunately, the quality of transported equine semen, and its subsequent fertility, appear to be extremely variable in private practice. Therefore, despite good fertility of the mare and adequate experience of the veterinarian coordinating the insemination with ovulation, the mare may not get pregnant due to the poor quality of semen that is received. This variability in semen quality may be due to the inherent ability of the stallion’s semen to withstand a particular cooling process; the expertise of the person collecting, processing, and preparing the semen for shipment; differences in transport containers; as well as events that occur with commercial couriers during the transport to the AI facility.

A retrospective study examined and determined the pregnancy rate per cycle that can be expected with semen shipped from many different sources into a private equine practice. Mare candidates were selected based on those presented to the breeding facility by owners. They were categorized based on reproductive status as maiden mares (14), barren mares (30), and foaling mares (nine). Fifty-three mares, of various breeds, were bred over 81 cycles with semen from 41 different stallions that was shipped by a commercial carrier. The semen arrived in either an Equitainer, a Bioflyte, an Expect-A-Foal, or a modified version of a shipping container. The mares were inseminated at least once within 48 hours preceding ovulation. The presence of a corpus hemorrhagicum was documented by ultrasonographic examination using a five MHz linear array ultrasound probe. Pregnancy was confirmed by the same method 12-18 days following ovulation or by the attainment of an embryo seven to eight days following ovulation.

The criteria used for the subjective evaluation of shipped semen included progressive motility estimation of the spermatozoa, morphology of the spermatozoa, total number of progressively motile spermatozoa, concentration of the semen, packaging of the semen, the container in which the semen was shipped, and accompanying paperwork that included a semen analysis, instructions for use, and adequate labeling of the semen. Out of 81 heat cycles in which mares were bred with shipped semen, 39 resulted in a pregnancy, thereby resulting in a 48% per cycle pregnancy rate. The heat cycles in which mares were inseminated with excellent versus good versus fair versus poor quality semen resulted in 87.5% (14/16), 62% (18/29), 33% (6/18), and 11% (2/18) pregnancy rates, respectively.

In this study, pregnancy rates in mares inseminated with shipped semen in private practice reflect the quality of the semen received. Many of the insemination dosages received during this study failed to meet the recommended insemination dose for maximal pregnancy rates. As previously reported, maximal pregnancy rates should be achieved with an insemination dose of greater than or equal to 500 X 106 progressively motile sperm within 48 hours preceding ovulation. This semen should be extended to a final concentration of 20-50 X 106 progressively motile spermatozoa/ml in order to maximize pregnancy rates.

The results of this study may be somewhat skewed by the variation of frequency in insemination. If sufficient semen was sent, mares participating in this study were often bred with a second dose of semen. In the case that two adequate insemination doses are included in the shipment, pregnancy rates may be enhanced by re-insemination on the following day if ovulation has not occurred, presumably due to the replenishment of storage sites that are responsible for selecting spermatozoa in the mare’s oviducts.

Results of this study demonstrate the importance of a pre-breeding/pre-shipment breeding soundness examination (BSE) of the stallion as well as proper handling, processing, and shipment of the ejaculate. It is important to realize that all transport containers are not the same. The cooling rates and ability to maintain storage temperatures of different shipping vessels may vary and thereby affect motility and subsequent fertility of the semen. Although Heiskanen et al. reported per cycle pregnancy rates of 87% in 26 mares bred over 30 cycles with semen that had been cooled for 40 hours from three stallions, it is generally accepted that fertility of semen is compromised to varying degrees with a cooling process more than 24-48 hours regardless of the transport container. Still, the low pregnancy rate per cycle of 48% in this study suggests that impaired fertility may be secondary to causes other than changes that occur with semen cooling. The causes of this compromise may range from improper handling of the semen to the inherent quality of the semen itself.

Many of the European registries limit their breeding stallion licenses based on a stallion’s BSE. Stallions within the United States are not selected or ranked based upon fertility, but rather upon performance alone. This selection process may be a disservice to all involved in the breeding industry. If fertility of the semen and the ability to maintain this fertility in shipping are indeed a heritable trait in the stallion, whole breeds may be potentiated carrying the ability or inability to withstand the rigors associated with shipping semen.

In conclusion, many breed registries have expanded their requirements for registration and subsequently, transportation and artificial insemination of equine semen have rapidly become routine practice. In order to maximize pregnancy rates of mares inseminated with shipped semen, the fertility of both the mare and stallion must be optimized as well as the expertise of the veterinarians processing the semen and coordinating insemination.


Metcalf, E.S., Pregnancy rates with cooled equine semen received in private practice. 44th Ann AAEP Conv Proc, p. 16-18, 1998.

Householder, D.D., Pickett, B.W., Voss, J.L., and Olar, T.T. Effect of extender, number of spermatozoa and hCG on equine fertility. Eq Vet Sci Jan/Feb, p. 9-13, 1981.

Jasko, D.J., Moran, D.M., Farlin, M.E., Squires, E.L., Amann, R.P., and Pickett, B.W. Pregnancy rates utilizing fresh, cooled and frozen-thawed stallion semen. 38th Ann AAEP Conv Proc, p. 649-660, 1992.

Squires, E.L., Brubaker, J.K., McCue, P.M., and Pickett B.W. Effect of sperm number and frequency of insemination on fertility of mares inseminated with cooled semen. Theriogenology 49 (4), p. 743-750, 1998.

Dobrinski, I., Thomas, P., Smith, T., and Ball, B. Sperm-oviduct interaction: role of sperm adhesion and effects of sperm cryopreservation. 42nd Ann AAEP Conv Proc, p. 144-145, 1996.

McKinnon, A.O., and Walker, J.B. Effect of ambient temperature and container on temperature of extended semen. World Eq Vet Review 3 (1), p. 5-11, 1998.

Heiskanen, M.L., Huhtinen, M., Pirhonen, A., Maenpaa, P.H. Insemination results with slow-cooled stallion semen stored for approximately 40 Hours. Acta Veternaria Scandinavica 35 (3): 257-262, 1994.

About the Author

Elizabeth Metcalf, MS, DVM

Elizabeth (Lisa) Metcalf, MS, DVM, a private practitioner from Sherwood, Ore., has been featured frequently at reproduction seminars.

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