The induction of oestrus in ewes during the non-breeding season using pre-used CIDRs and oestradiol-17β treatment

The fertility obtained in sheep after the use of intravaginal progesterone devices is related to the content of progesterone of the device. The hypothesis of this study was that the reproductive response of anoestrous ewes to the ram-effect could be improved by the administration of oestradiol-17β in conjunction with CIDRs treatment--using previously used CIDRs in a 5-day progestagen priming. Therefore, the objective was to determine if oestradiol-17β treatment increases fertility of anoestrous ewes primed with used CIDRs and stimulated by the ram-effect. The hypothesis was tested with CIDRs that had been previously used for 12 or 18 days. The trial was performed during the non-breeding season using 158 Corriedale ewes. Ewes had been isolated from rams since Day -35 (Day 0=introduction of the rams). A CIDR (0.3g progesterone, InterAg, Hamilton, New Zealand) was inserted on Day -5 in all ewes with CIDR that had been previously used for 12 days (n =62) or 18 days (n =96). Also on Day -5, 29 and 53 ewes that had received CIDRs of 12 or 18 days, respectively, received an intra-muscular treatment of 50μg of oestradiol-17β (E groups). The ewes that did not receive the oestradiol-17β treatment remained as the control group (C group). Overall the treatment groups were thus: C12 (n =33), C18 (n =43), E12 (n =29), and E18 (n =53). On Day 0 all CIDRs were withdrawn, and ewes were placed with 18 rams and 20 ewes hormonally induced to exhibit oestrus. Sexual receptivity of ewes treated with CIDRs was estimated from marks on the rumps of the ewes daily from Day 0 to Day 5, and the pregnancy status diagnosed with transrectal ultrasonography on Day 40. The percentage of ewes exhibiting oestrus and pregnancy rates were lower in ewes synchronized with previously used CIDRs for 18 days, compared to those used for 12 days. The responses of ewes in oestrus were 39.4, 14.0, 65.5, and 32.1% for the C12, C18, E12, and E18 groups respectively, with pregnancy rates of 30.3, 14.0, 34.5, and 17.0%. Administration of oestradiol-17β increased the frequency of oestrous response in ewes that were treated with CIDRs previously used for 12 days (P <0.05), but not in those treated with CIDRs used for 18 days. It could be concluded that the administration of oestradiol-17β only improved the percentage of ewes responding to oestrus when CIDRs previously used for 12 days were used for 5 days before the introduction of rams. No positive effect on fertility was observed irrespective of the period during which CIDR had been previously used.