Reproductive outcome with GnRH inclusion at 24 or 36 h following a prostaglandin F2α-based protocol for timed AI in ewes

The objective of this experiment was to study the reproductive performance obtained after a short-interval prostaglandin (PG) F2α-based protocol for timed artificial insemination (TAI) in sheep (Synchrovine®: two injections of PG 7 d apart), including a GnRH analogue at 24 or 36 h after the second PG injection. The experiment involved 296 Corriedale ewes (206 multiparous and 90 nulliparous) grazing natural pastures during the breeding season (March-April; UTU “La Carolina”, Flores Uruguay, 33° S-57° W). Ewes were assigned to three treatment groups: a) Synchrovine® (Control, n = 101): two injections of D-Cloprostenol 75 μg, 7 d apart, b) Synchrovine® + GnRH24 (n = 98): Synchrovine® plus GnRH (busereline acetate 8.4 μg) 24 h after the second PG injection, and c) Synchrovine® + GnRH36 (n = 97): Synchrovine® plus GnRH 36 h after the second PG injection. All ewes were subjected to cervical TAI (Day 0), 44 to 47 h after second PG injection, with fresh extended semen pool from six rams. Reproductive performance of ewes having ovulations and ovulation rate on Day 10, estrous cycle length in ewes that returned to estrus and non-return rate to estrus up to Day 22, fertility, prolificacy and fecundity on Day 70 were analyzed. Ewes having ovulations, ovulation rate, estrous cycle length and prolificacy did not differ between groups (P > 0.05). However, non-return to estrus, fertility and fecundity was decreased in Synchrovine® + GnRH24 (P < 0.05) and similar between Synchrovine® and Synchrovine® + GnRH36 (P > 0.05). It was concluded that the reproductive performance obtained by Synchovine® TAI protocol was impaired by GnRH at 24 h and not improved by GnRH administered at 36 h after the second PG injection.