Comparison of pregnancy rates using new versus once used CIDR in 7 day estrus synchronization protocol during breeding season in ewes

Small ruminant CIDRs are labelled for use in estrus synchronization protocols for up to 14 days. Protocols of shorter duration (5 - 9 days) include prostaglandin F 2α (PGF2α ) treatment at CIDR removal to induce luteolysis. Short duration protocols have led to anecdotal belief that a CIDR may be used a second time. Addition of gonadotropins to CIDR based protocols are known to enhance follicular growth and facilitate multiple ovulations. Our objective was to compare pregnancy status following natural breeding at a synchronized estrus using a 7 day exposure to either a new CIDR or once used CIDR, with PG600 treatment on day 6. Canadian Arcott ewes (mixture of lambs and mature) housed at 2 separate locations (Flocks A and B) were used. In Flock A, ewes (n = 61) were synchronized during 2018 - 2019 (n = 31) and 2019 - 2020 (n = 30) breeding seasons, whereas ewes (n = 71) in Flock B were synchronized during 2018 - 2019 breeding season. On day 0, ewes were randomly allocated to have either a new CIDR (nCIDR) or used CIDR (uCIDR) placed intravaginally. On day 6, all ewes were treated with PG600 (400 IU eCG + 200 IU of hCG/5 ml dose, IM). On day 7, CIDRs were removed and all ewes received a single dose of PGF 2α (cloprostenol, 125 mg, 0.5 ml, IM). On day 8, ewes were exposed to rams for 4 days. Rams were identified as satisfactory breeders using breeding soundness evaluation and previous breeding history. Ram to ewe ratio was maintained at 1:5 (Flock A) and 1:5 to 1:6 (Flocks B). Rams were provided 4 - 5 days of sexual rest before being used. Pregnancy diagnosis was performed between days 45 - 60 postbreeding via transabdominal ultrasonography. Fischer’s Exact test was used to compare pregnancy rates between treatment groups. All ewes in Flock A were marked (ram marking harness) within 24 hours after ram introduction. Overall, pregnancy rates (nCIDR and uCIDR combined) were different (p < 0.01) between flocks (85%, 52/61 versus 58%, 41/71; Flocks A versus B, respectively); therefore, data were analyzed separately by flock. Within flock, pregnancy rates were not different between treatment groups (Flock A: nCIDR versus uCIDR, 25/30 versus 27/31, p = 0.75; Flock B: nCIDR 21/34 versus 20/37, p = 0.63). Lower pregnancy rates in Flock B were believed to be due to nutritional status (as a general observation, body condition scores of ewes in Flock B were lower than those in Flock A). In conclusion, a short duration (< 7 days) once used CIDR, can be successfully reused to obtain pregnancy rates comparable to new CIDR.

Keywords: Estrus synchronization, PG600, CIDR, ewes, fertility

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.