Effect of GnRH and hCG Treatment Following a Short-Term Estrus Synchronization Protocol on Ovulation and the Fertility in Merino Ewes, During the Breeding Season

This study aimed to compare the effect of hCG or GnRH administered 36 h following a short-term estrus synchronization treatment on the ovulation time and pregnancy rates of Merino ewes during the breeding season. The estrus cycles of ewes were synchronized with an intravaginal sponge containing 60 mg of medroxyprogesterone acetate for 6 days, and an injection of 400 IU of eCG and 125 µg of d-cloprostenol 24 h before sponge removal. Thirty-six h after the sponge removal, ewes were injected intramuscularly either 1 ml of physiological saline solution (control-group; n=14), 100 IU of hCG (hCG-group; n=14) or 0.004 mg of buserelin acetate (GnRH-group; n=14). The estrus behavior was observed using teaser rams and the ovulation time was monitored using transrectal ultrasonography twice daily for 96 h after the sponge removal. Ewes in estrus were allowed a single mating using fertile rams. Estrus response was higher (P<0.05) in the control group (92.86%) than in the GnRH group (50.00%). The interval from sponge removal to ovulation and from hCG to ovulation were shorter in the control group than in the hCG group (70.55, 80.83 h; 34.55, 44.83 h, respectively, P<0.05). In conclusion, administration of hCG or GnRH at 36 h following a short-term estrus synchronization protocol did not affect estrus behaviors, ovarian findings and pregnancy rates in ewes during the breeding season.