Efficacy of various five-day estrous synchronization protocols in sheep

The objective of this study was to determine the effects of ewe estrus synchronization protocols including combinations of controlled internal drug releasing (CIDR) inserts for 5 d, GnRH, and PG on concentrations of progesterone (P4) and reproductive performance of ewes during different breeding seasons. Experiment 1 was conducted during the transition season (August-October) and Experiment 2 was conducted during the anestrous season (April-May). In both experiments ewes received 1 of 4 treatments: (1) untreated (CON); (2) CIDR (0.3 g P4) insert (CT); (3) CIDR and PG at CIDR removal (CTP); or (4) GnRH at CIDR insertion and PG at CIDR removal (CTG). Rams equipped with marking harnesses were introduced at CIDR removal (d 0) and ewes were observed for breeding marks twice daily at 0800 h and 1700 h. In Experiment 1, days to estrus was greater (P ≤ 0.01) in CON (9.5 ± 1.10 d) and CTP (6.5 ± 1.04 d) compared with CT (3.6 ± 1.03 d). No differences (P = 0.27) were detected in pregnancy rate within the first 17 d of the breeding season for CON, CT, CTP, and CTG treatments (37 ± 7.8%, 62 ± 7.5%, 56 ± 8.1%, and 46 ± 7.7%, respectively). Similarly, no differences (P ≥ 0.28) were detected among treatments for lambing rate and prolificacy. Contrasts for all CIDR treatments compared to controls determined that CIDR treated ewes had less days to estrus and lambing (P ≤ 0.05) than control ewes. In Experiment 2, days to estrus was greater (P ≤ 0.02) in CON (6.5 ± 1.05 d) and CTP (5.9 ± 0.88 d) compared with CTG (3.0 ± 0.88 d). No differences were detected (P ≥ 0.23) among treatments for overall percentage of ewes exhibiting estrus. No differences (P = 0.79) in pregnancy rate within the first 17 d of the breeding season were detected among treatments (42 ± 12.4%, 37 ± 11.3%, 35 ± 11.1%, and 50 ± 10.9% for CON, CT, CTP, and CTG, respectively). No differences (P ≥ 0.44) were observed among treatments in overall lambing rate and prolificacy. In conclusion, inserting a CIDR for 5 d prior to ram introduction regardless of the use of PG or GnRH, reduced the interval to estrus compared with untreated ewes but did not impact overall pregnancy rates, lambing rates, or prolificacy.