Time of insemination
The time of insemination effects both the pregnancy rate and embryonic loss. Insemination of fresh or cooled semen is most efficient before ovulation. Pregnancy rates and embryonic loss after insemination of fresh semen relative to the day of ovulation is shown in the graph below. Insemination before the day of ovulation results in an increasing pregnancy rate (highest on the day before ovulation: 88%) and a low rate of embryonic loss (14%). Pregnancy rate was lower when mares were inseminated on the day of ovulation (55%) and the embryonic loss rate increased (34%). Insemination after ovulation results in a decrease in pregnancy rate (7%) and an increase in early embryonic loss (33%). These results indicate that mares need to be inseminated prior to ovulation to allow sperm time to reach the site of fertilization and undergo capacitation. If ovulation occurs before sperm complete these events, then the oocyte sits within the oviduct and waites until the sperm are present and ready to fertilize the oocyte. Mean while, the oocyte undergoes a detrimental ageing effect that leads first to embryonic death and with more ageing, reduced fertilizability.
The effect of inseminating a mare after ovulation is shown even more dramatically in the graph below. Pregnancy rate when mares are inseminated after ovulation decreases rapidly. By 30-36 hours post ovulation, pregnancy no longer occurs (remember that the mare remains in behavioral estrus for about 24-48 hr after ovulation).
