Frozen vs. Cooled Semen

Q. We are getting ready to breed our mare to a stallion whose semen is only shipped frozen. Does using frozen-thawed semen instead of cooled semen reduce the chances of our mare getting pregnant?


A. Generally, any time you manipulate semen, the per-cycle pregnancy rate decreases. The rate should be highest for fresh semen, then shipped semen, followed by frozen-thawed semen. This assumes a reproductively healthy mare and stallion. Per-cycle pregnancy rates with frozen-thawed semen are reported to range from 0 to 70%.

Therefore, it is important to do a little homework prior to choosing to use frozen semen. It is important to know the first-cycle pregnancy rate for that stallion's frozen-thawed semen. This information is also important when using shipped-cooled semen. Next one needs to know how the mares were bred to achieve this pregnancy rate. Were the mares bred before, after, or before and after ovulation? Also, were the mares bred with a full dose (and what constitutes a full dose for that particular stallion) or was a dose split in half to allow breeding before and after ovulation? Was the semen deposited in the uterine body or was deep horn insemination used?

In order to achieve the stallion's reported pregnancy rate, you must be able to repeat the breeding management used to obtain those rates.

Mare selection also plays a role in the success or failure of frozen thawed semen. A reproductively healthy mare should always be used when fertilizing with frozen-thawed semen. Older maiden mares should be avoided because the cervix is not able to relax properly in many of these mares. There is an inflammatory response to insemination with frozen-thawed semen. One aspect of an inflammatory response is fluid production by the uterus. If the cervix does not relax properly to allow the fluid to escape, the inflammation increases (as does fluid production and accumulation), and a vicious cycle begins. There is more effort and cost associated with the use of frozen-thawed semen, but this can be decreased if multiple doses of semen are available. In conclusion, success with frozen-thawed semen is stallion- and mare-dependent. If you have a reproductively sound mare and can find a stallion that has good fertility with his frozen-thawed semen, you can be successful.

About the Author

Pete Sheerin, DVM, Dipl. ACT

Pete Sheerin, DVM, Dipl. ACT, is a practitioner at the Rood and Riddle Equine Hospital in Lexington, Ky.

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