Factors affecting pregnancy rates in mares bred with cryopreserved semen

Historically, 8 × 0.5 ml straws, containing approximately 800 million sperm and 250 million progressively motile sperm were provided as a single ‘breeding dose’ of cryopreserved stallion semen. With the use of deep horn artificial insemination, there is a trend to reduce the number of 0.5 ml straws sold as a breeding dose, sometimes down to as little as one straw. Our aims were to determine if the number of straws provided as a breeding dose, as well as other mare, stallion and management factors, have an impact on pregnancy outcome in mares inseminated with cryopreserved semen. Unexpectedly, we identified no effect of the number of 0.5 ml straws on pregnancy outcome. We also identified no difference in pregnancy outcome for those mares inseminated once post-ovulation compared to mares inseminated once pre- and once post- ovulation. Additionally, for mares inseminated once post-ovulation, we identified no benefit of breeding 0-3 hours post-ovulation vs. breeding 0-6 hours post-ovulation. Other factors not associated with pregnancy outcome included: whether an endometrial sample was obtained for bacteriologic culture, whether the endometrial sample produced bacterial growth, whether a mare developed fluid after breeding, whether a mare was treated for bacterial endometritis and/or uterine fluid, and post-thaw progressive sperm motility. These results suggest the existence of an effective industry self-selection process in which only semen from the most fertile stallions is marketed in these ‘ultra-low’ doses and that breeding mares within 3 hours post- ovulation provides no benefit to pregnancy outcome compared to breeding mares within 6 hours post-ovulation.