Fertility of mares after postovulatory insemination

In a first experiment, 11 Finnhorse mares were examined every six hours during late oestrus by rectal palpation and ultrasonography to determine the time of ovulation. The mares were inseminated over one to three subsequent cycles, 6-12 (n = 5), 12-18 (n = 5), 18-24 (n = 5) and 24-30 (n = 5) hours after ovulation. Pregnancies were terminated by prostaglandin injection 21 days after insemination. All mares inseminated within 18 hours of ovulation conceived but no mare inseminated 24 hours or more after ovulation conceived. In a second experiment, 14 mares were examined every day at about the same time and inseminated after ovulation had been detected, within 0-27 hours of ovulation. Pregnancies were terminated by prostaglandin injection 25 days after insemination and the animals inseminated again. Forty-six per cent of the mares (13 out of 28) were pregnant 16 days after insemination but there was early embryonic death in five mares, i.e. only 29% of the mares became and remained pregnant. The equine ovum seems to be capable of being fertilized for at least 18 hours after ovulation. However, the high incidence of early embryonic death in the second experiment suggests that the viability of the embryo may have decreased.