The Effect of Time of Breeding Relative to Ovulation on Pregnancy Rate When Using Cooled Transported Semen or Natural Mating in the Mare

• Cooled semen must be inseminated within 24 hours before and 12 hours after ovulation for highest pregnancy rate (PR).
• Pregnancy rates of mares bred by natural mating are comparable to that of mares inseminated with cooled semen only within 24 hours of ovulation.
• Postovulatory breeding within 12 hours of ovulation yielded similar PRs in mares inseminated with cooled semen or by natural mating.

This retrospective study analyzed data from 505 inseminations with cooled transported semen and 387 natural matings. The analysis shows that similar (P > .05) per cycle pregnancy rates (PRs) can be achieved with cooled transported semen equal to that with natural mating, provided that insemination is performed between 24 hours before and 12 hours after ovulation. Any earlier inseminations relative to ovulation yielded lower PRs (P < .05) than in mares bred by natural mating.