Effects of estradiol and oxytocin injection on the efficiency of artificial insemination in Iranian Zel ewes during the breeding season

In sheep industry, pregnancy rate after artificial insemination (AI) declines due to the complex anatomy of the cervix in ewes, such that it might prevent effective intrauterine insemination. At estrus, cervical relaxation occurs to some degree in ewes, which is regulated by the changes in the levels of reproductive hormones. This study aimed to evaluate the effects of estradiol and intravenous (IV) or intramuscular (IM) oxytocin injection at different doses on the cervical opening and pregnancy rate of Iranian Zel ewes during the breeding season. For this purpose, three experiments were conducted on 120 ewes (3-4 years old, weighing 47±2.5 kg). In the first experiment, ewes were equally assigned to two groups to receive estradiol (100-200 µg). After 12 h, each group was equally divided into six subgroups (n=20) and received 50, 100 and 150 IU oxytocin via IV and IM injection. Cervical opening was measured before and 15 min and 12 h after estradiol injection and 20 min after oxytocin administration. In the second experiment, we only assessed the effect of oxytocin administration on cervical opening similar to the first experiment. In the third experiment, controlled internal drug release (CIDR) was used in all the ewes for 12 days to induce estrus synchronization. Afterwards, the ewes received 550 IU intrauterine equine chorionic gonadotropin at the time of CIDR removal. Before AI, ewes were equally categorized into three groups (n=40); the first group was considered as control, and the other two groups received 100 IU oxytocin via IM or IV injection. At 54 h after CIDR removal, all ewes were inseminated transcervically using diluted fresh semen. Pregnancy was detected via ultrasound 50 days after insemination, and lambing and twinning rates were measured after parturition. Results of the first and second experiment indicated that estradiol injection had no effect on cervical opening (P>0.05), while the administration of 100 or 150 IU oxytocin (IV or IM) could dilate the cervix with or without estradiol (P