Beef Species: Beef Management

The objective of this study was to quantify fixed-time AI (TAI) pregnancy rates resulting from reduced intervals between 2 25 mg dinoprost tromethamine (PG) injections on d 5 of a 5-d CIDR-Co-synch estrus synchronization program. Angus and Angus-influenced commercial cows (n = 873) maintained on native pasture at 3 locations were randomly assigned to treatments, blocking for BCS and postpartum interval (PPI). On experimental d 0 all cows received 100 μg GnRH i.m. and a CIDR (1.38 g progesterone). On d 5, CIDRs were removed and cows were administered 2 injections of 25 mg PG i.m. either 2, 4 or 6 h ± 30 min apart. On d 8, 72 ± 3 h after the first PG injection, all cows received 100 μg GnRH i.m. and were TAI. Conception rates were determined by rectal ultrasonography 40 ± 2 d following TAI. The TAI conception rate was 59.7% for the 6 h interval group, 51.4% for the 4 h interval group (P = 0.05), and 50.6% for the 2 h interval group (P = 0.04). Body condition score (P = 0.06) was a marginally significant source of variation for TAI conception rate; shorter PPI intervals negatively affected pregnancy rate (P = 0.02). Reducing the interval between PG injections from 6 to 4 or 2 h resulted in a 9-percentage point (15%) reduction in TAI conception rates for cows in this 5-d CIDR-Co-synch estrus synchronization protocol.