A study performed by Utrecht University in the Netherlands looked at the reproductive records of 9,225 mares over a five-year period. The mares were bred to 88 different stallions, either natural cover or by artificial insemination.
Success of breeding was defined as the mare not being returned for rebreeding within 28 days after the end of her last estrus period. The significant findings were:
• Conception rates for fresh semen ranged from 67 to 74%; for frozen/thawed semen 59%.
• Of mares that did not take on the first breeding, those that required breeding only twice had a better chance of pregnancy than those which returned three or more times. It’s not clear if the reason is that mares bred more than once may not have been bred at the optimal time for conception, or if the repeated inseminations themselves create conditions that are not optimal for conception.
• Mares bred before May 1 had a better chance of becoming pregnant than those bred after July 1.