FERTILITY OF SUPEROVULATED EWES FOLLOWING INTRAUTERINE OR OVIDUCAL INSEMINATION WITH FRESH OR FROZEN-THAWED SEMEN

Two experiments were conducted with mature Merino ewes to investigate the effects of time and site of insemination of fresh and frozen-thawed semen on the fertility of superovulated ewes. In Experiment 1, each ewe was treated with an intravaginal progestagen sponge and PMSG and/or FSH. They were inseminated in the uterus with fresh or frozen-thawed semen (approximately 100 x 10(6) motile sperm) at 24, 44 or 64 h after sponge withdrawal. Ova were recovered at 88 h after sponge withdrawal and classified as fertilized if they had pronuclei or had cleaved. Mean fertilization rates of recovered ova were 60.0, 93.7 and 87.8% for fresh semen and 46.1, 98.2 and 26.1% for frozen-thawed semen at each of the insemination times (24, 44 and 64 h) respectively. Overall, fertilization rates were higher for fresh semen than for frozen-thawed semen (P < 0.01), but there was an interaction with time of insemination (P < 0.01). Following insemination with frozen-thawed semen at 64 h, only 61% of the fertilized ova developed to the 2- to 4-cell stage by the time of embryo recovery; this was less than in any of the other groups (P < 0.05). In Experiment 2, ewes were inseminated with 100 x 10(6) motile fresh or frozen-thawed semen in the uterus or in the oviducts at 64 h after sponge withdrawal. Fertilization rates following intrauterine insemination at 64 h were lower for frozen-thawed semen than for fresh semen (19.8 v. 87.1%, P < 0.01), but there was no difference between fertilization rates following oviducal insemination with fresh or frozen-thawed semen (95.4 v. 94.5%). The low fertilization rates achieved for late uterine insemination with frozen-thawed semen appear to be due to inefficient transport of the frozen-thawed spermatozoa into, or within, the oviducts.