Effects of time of insemination relative to ovulation on pregnancy rate and embryonic‐loss rate in mares

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Abstract
Summary: The effects of pre‐ovulatory and post ovulatory insemination on pregnancy rate and embryonic‐loss rate were studied in 268 mares in two experiments. Within each experiment mares were randomised within replicates as follows: to be inseminated on the day the pre‐ovulatory follicle reached 35 mm (pre‐ovulatory group), to be inseminated on the day of ovulation (Day 0 group), and to be inseminated on the day after ovulation (Day 1 group). Ultrasonic pregnancy diagnoses were performed on Days 11, 12, 13 and 14 (Experiment 1) and Days 11, 12, 13, 14, 15, 20 and 40 (Experiment 2). Combined for the two experiments, pregnancy rates were different (P<0.01) among the three groups. For Experiment 2, pregnancy rate within the pre‐ovulatory group was lower (P<0.05) for insemination 4 days or more before ovulation than for up to 3 days before ovulation. Pregnancy rate was lower (P<0.05) for the Day 0 group than for the pre‐ovulatory group inseminated up to 3 days before ovulation. In Experiment 2, ovulation was detected by examinations every 6 h; pregnancy rate was greater (P<0.05) for mares inseminated 0 to 6 h after ovulation than for mares inseminated at 18 to 24 h. No pregnancies occurred when mares were inseminated 30 h or more after ovulation. The mean day of first detection of the embryonic vesicle was different (P<0.0001) among the three groups. Diameter of embryonic vesicle averaged over Days 11 to 15 also differed (P<0.0001) among groups. The rate of embryonic loss (Experiment 2) between day of first detection of the conceptus and Day 40 was greater (P<0.05) for the Day 0 group than for the pre‐ovulatory group. Insemination up to 12 h post ovulation resulted in pregnancy rates equivalent to those for pre‐ovulatory insemination. Thereafter, the pregnancy rates diminished with no pregnancies occurring after 30 h. Embryonic vesicles were smaller and embryonic‐loss rate was greater for mares inseminated after ovulation.