The effects of different insemination regimes on fertility in mares.

This research explores how different artificial insemination (AI) regimes impact the pregnancy rates in horses, specifically analysing the effect of the number of inseminations, the time interval between them, and their proximity to ovulation. The study concluded that a single insemination per cycle yields pregnancy rates equivalent to multiple inseminations, provided it is done within specific time frames relative to ovulation, dependent on the type of semen used.

Objective and Approach

• The primary goal of this study was to investigate how different AI regimes affected fertility rates in mares, examining factors like the number of inseminations each oestrus, the time difference between inseminations within the same oestrus, and how close the insemination is to ovulation.
• The study utilized two separate experiments. The first one involved 401 warmblood mares and examined their response to one to three inseminations within each oestrus. The second experiment was a retrospective analysis of the breeding records of 2,637 mares over 5,305 oestrous cycles during the 1999 breeding season.

Experiment 1

• In the first experiment, follicle development was monitored using a transrectal ultrasonographic examination of the ovaries every 12 hours until ovulation occurred.
• Mares were artificially inseminated with either cooled or frozen-thawed semen from fertile Hanoverian stallions, beginning at intervals ranging from 24 hours before to 36 hours after human chorionic gonadotrophin (hCG) administration.
• Mares were injected intravenously with 1500 IU hCG when they were in oestrus and had a pre-ovulatory follicle greater than or equal to 40mm in diameter.
• Pregnancy detection was performed sonographically 16-18 days after ovulation.
• The results demonstrated ovulation occurred within 48 hours of hCG administration in an overwhelming majority of the mares (around 97.5%), and the interval between the hCG treatment and ovulation was significantly shorter later in the breeding season than earlier.

Experiment 2

• In the second part of the study, which was retrospective, researchers looked at insemination data and related it to the number of live foals registered the next year.
• While the first experiment found that the number of inseminations had no significant impact on pregnancy rates when tied to specific ovulation times, the second experiment showed that mares inseminated with cooled semen once per cycle had significantly lower rates of foaling than those inseminated two or more times.
• There was also a tendency for foaling rates to increase when mares were inseminated daily as opposed to every other day until ovulation.

Conclusions

• The study concluded that under conditions of frequent veterinary examination, a single insemination per cycle is as effective as multiple inseminations, provided that it is performed between specific time frames prior to and after ovulation, depending on whether cooled or frozen-thawed semen is used.
• However, if regular scanning is not possible, an optimal fertility rate appears to be achieved by daily insemination until ovulation.