Factors affecting pregnancy rates in mares bred with cryopreserved semen.

Overview

• This research investigated whether the number of frozen semen straws used per breeding dose and various mare, stallion, and management factors influence pregnancy rates in mares.
• The study found no significant effects of the number of insemination straws or timing of breeding relative to ovulation on pregnancy outcomes in mares inseminated with frozen semen.

Background

• Traditional breeding doses of cryopreserved stallion semen consist of 8 straws (0.5 ml each), containing approximately 800 million sperm and 250 million progressively motile sperm.
• With advancements such as deep horn artificial insemination, the number of straws used per breeding dose has decreased, sometimes to a single straw.
• The impact of reducing straw numbers on pregnancy success rates was unclear and formed the basis for this study.

Research Aims

• To determine whether the number of frozen semen straws used as a breeding dose affects pregnancy rates in mares.
• To evaluate the effects of other factors including mare characteristics, stallion fertility, breeding timing (before or after ovulation), and semen quality on pregnancy outcomes.

Methods and Variables Studied

• Comparison of pregnancy outcomes in mares bred with varying numbers of 0.5 ml straws of cryopreserved semen.
• Examination of timing of insemination relative to ovulation: once post-ovulation versus once pre- and once post-ovulation.
• Comparison of insemination timing windows post-ovulation: within 0–3 hours versus within 0–6 hours.
• Other investigated factors included:
  • Whether an endometrial sample was collected for bacterial culture.
  • Growth of bacteria from endometrial cultures.
  • Development of uterine fluid after breeding.
  • Treatment of mares for bacterial endometritis or uterine fluid accumulation.
  • Post-thaw progressive sperm motility of the cryopreserved semen.

Key Findings

• No significant difference in pregnancy rates was found based on the number of 0.5 ml straws used as the breeding dose.
• No difference in pregnancy outcome between mares inseminated once after ovulation compared to those inseminated once before and once after ovulation.
• For mares inseminated once post-ovulation, breeding within 3 hours versus within 6 hours did not impact pregnancy success.
• Additional factors such as bacterial culture results, presence of uterine fluid, treatment for infections, and post-thaw sperm motility were not associated with pregnancy outcomes.

Interpretation and Implications

• The findings suggest a robust self-selection process in the equine breeding industry, where only semen from stallions with high fertility is marketed in very low insemination doses (“ultra-low” doses), maintaining pregnancy rates despite reduced sperm numbers per dose.
• This challenges previous assumptions that higher sperm numbers or multiple inseminations are necessary to maximize pregnancy in mares inseminated with frozen semen.
• The results indicate flexibility in insemination timing post-ovulation without compromising pregnancy rates, potentially easing management protocols for breeders.
• Other clinical factors commonly considered critical for pregnancy success, like uterine infections or fluid accumulation, did not show an effect in this study’s population, possibly reflecting good clinical management or mare selection.

Conclusions

• Reducing the number of frozen semen straws to as few as one per breeding dose does not adversely affect mare pregnancy rates when using deep horn artificial insemination.
• Insemination timing within 6 hours post-ovulation is as effective as within 3 hours, offering practical flexibility for breeding management.
• The equine breeding industry appears to effectively select and market highly fertile stallion semen suitable for successful breeding even at minimal doses.
• Overall, these insights support efficient and potentially cost-saving breeding strategies without sacrificing reproductive success in mares bred with cryopreserved semen.