Frozen semen can be expensive depending on the number of straws used as a breeding dose. In a recent study, Pasch et al. examined whether the number of straws affects breeding success and if other mare and stallion factors impact pregnancy outcome with frozen semen. Maria Schnobrich, VMD, DACT, of Rood & Riddle Equine Hospital, brought this paper to the Kester News Hour at the 2024 American Association of Equine Practitioners’ Convention in hopes that practitioners only have to inseminate once rather than pursue multiple time-consuming and exhausting follow-ups during a mare’s ovulation. This 11-year retrospective study involved 624 estrous cycles, 277 fertile stallions, and 473 known pregnancy outcomes. The mares’ median age was 10.
Insemination Protocol
Mares were bred using a deep horn insemination procedure. For a single insemination protocol, mares were evaluated 10-16 hours after inducing ovulation with deslorelin IM and then approximately every eight hours. The single insemination occurred within a 0-6-hour post-ovulation window. For the timed insemination protocol, the mares were checked 12 hours following deslorelin IM administration for ovulation. (Timed insemination relies on insemination at a predetermined time based on a mare’s ovulation cycle following deslorelin administration. The mare is inseminated within a “standard” 12 hours prior to and/or six hours after ovulation.)
Factors the researchers examined included reproductive tract findings such as uterine edema, fluid, and bacteriologic culture results. They also considered the number of straws used for insemination, the number of ovulations, the timing of ovulation to insemination, whether there was one or two inseminations, and sperm motility post-thaw.
Most–84%–of the inseminations were single dose following ovulation. The median timing of ovulation occurred 37.5 +/- 11.6 hours following deslorelin IM. Reproductive tract testing showed low bacterial growth in 25%.
Pregnancy Rate
The pregnancy rate was high, even when the statistics were broken down by age and included mares over 20. There was no significant difference between timed insemination and a single dose with regard to pregnancy rates. Pregnancy rates varied between 56% and 74%, with a single post-ovulation insemination resulting in a 63% pregnancy rate.
Other factors that did not affect pregnancy rate included:
- Presence of bacterial growth.
- Treatment—with dexamethasone, oxytocin, uterine lavage—prior to or after breeding.
- Intrauterine fluid after breeding, which is a marker for inflammation.
- Whether a mare was followed every three hours or six hours did not make a difference when using one dose of semen.
- Using four or fewer straws did not make a difference for single vs. timed insemination.
- Post-thaw motility.
Final Thoughts
In summary, timed insemination with two doses is just as likely to result in pregnancy as a single post-ovulation insemination. Neither the number of straws used nor post-thaw motility affect pregnancy outcome. In conclusion, Schnobrich said veterinarians might not need to check mares every three hours; six-hour checks are OK for post-ovulation, deep horn insemination with frozen semen.
Reference
Pasch L, Stefanovski D, Dobbie T, Lewis G, Turner RM. Factors affecting pregnancy rates in mares bred with cryopreserved semen. J Equine Vet Sci. 2024 Oct;141:105167. doi: 10.1016/j.jevs.2024.105167. Epub 2024 Aug 14. PMID: 39151810
Related Reading
- UC Davis First to Produce Equine Embryos by In Vitro Fertilization With Frozen Sperm
- Disease Du Jour: OPU and ICSI in Horses
- Frozen-Thawed Semen Protocol Produces Satisfactory Fertility in Horses
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