Evaluation of the 14-d CIDR-PG and 9-d CIDR-PG Protocols for Synchronization of Estrus in Bos indicus-Influenced and Bos taurus Beef Heifers.

Two long-term estrus synchronization protocols were evaluated among Bos indicus-influenced and Bos taurus beef heifers on the basis of estrous response and pregnancy rate resulting from fixedtime artificial insemination (FTAI). Estrus was synchronized for 1139 heifers in three locations, and heifers were assigned to one of two treatments within each location based on reproductive tract score (RTS; 1-5 scale). Heifers assigned to the 14-d CIDRPG protocol received a controlled internal drug release (CIDR) insert (1.38 g progesterone) on Day 0 with removal on Day 14, administration of prostaglandin F2α (PG; 25 mg im) on Day 30, and administration of gonadotropin-releasing hormone (GnRH; 100 µg im) concurrent with FTAI 66 h after PG. Heifers assigned to the 9-d CIDR-PG protocol were administered PG concurrent with CIDR insertion on Day 5, PG concurrent with CIDR removal on Day 14, PG on Day 30, and GnRH concurrent with FTAI 66 h after PG. Estrus detection aids were applied at CIDR removal and PG to evaluate estrous response. Mean RTS differed (P < 0.0001) based on biological type, with higher rates of estrous cyclicity (RTS 4 and 5) among Bos taurus heifers (72%) than Bos indicus-influenced heifers (27%). The proportion of heifers expressing estrus following CIDR removal was greater (P = 0.01) among heifers assigned to the 14-d CIDR-PG (88%) compared to the 9-d CIDR-PG protocol (83%). Estrous response following CIDR removal was higher (P < 0.0001) among Bos taurus (95%) compared to Bos indicus-influenced (75%) heifers. Rate of estrous response prior to FTAI did not differ based on treatment but was higher (P < 0.0001) among Bos taurus heifers (60%) than Bos indicus-influenced heifers (45%). However, biological type did not affect estrous response when RTS was included in the model, as RTS affected (P < 0.0001) the rate of estrous response at CIDR removal and prior to FTAI. Heifers that expressed estrus prior to AI achieved higher (P < 0.0001) AI pregnancy rates than heifers failing to express estrus. Pregnancy rates to FTAI did not differ significantly based on treatment in either biological type. Higher rates of estrous cyclicity among Bos taurus heifers resulted in higher FTAI pregnancy rates among Bos taurus (51%) compared to Bos indicus-influenced heifers (39%). However, pregnancy rates of respective RTS did not differ based on biological type. In summary, long-term CIDR-based protocols provide a simple, effective method of estrus synchronization in Bos indicus-influenced and Bos taurus beef heifers. [ABSTRACT FROM AUTHOR]