Two doses of prostaglandin improve pregnancy rates to timed-AI in a 5-day progesterone-based synchronization protocol in beef cows

The objective was to compare reproductive performance of Angus-cross beef cows synchronized with GnRH, a progesterone-based intravaginal insert (Controlled Internal Drug Release, CIDR) for 5-d, and one dose of either dinoprost (PGF) or cloprostenol (CLP, a PGF analogue) or two doses of PGF on the day of CIDR withdrawal. All cows (N = 830) at six locations received 100 μg of GnRH and a CIDR on Day 0. Within farm, cows were randomly allocated to receive 25 mg of PGF at the time of CIDR insert removal on Day 5 (1 × PGF; N = 277), two 25 mg doses of PGF, the first given on Day 5 at the time of CIDR removal and the second 7 h later (2 × PGF; N = 282), or 500 μg of CLP at the time of CIDR removal on Day 5 (1 × CLP; N = 271). All cows were given 100 μg of GnRH on Day 8 (72 h after CIDR removal) and concurrently inseminated (5-d CO-Synch + CIDR). Cows were fitted with a pressure-sensitive estrus detection device at the time of CIDR withdrawal. Timed-AI pregnancy rates were greater (P < 0.0001) in the 2 × PGF (69.0%) than the 1 × PGF (52.0%) and 1 × CLP (54.3%) treatments. However, breeding-season pregnancy rates were not different among treatments (87.0% for 1 × PGF, 92.9% for 2 × PGF and 87.5% for 1 × CLP; P > 0.1). In conclusion, cows that received two doses of PGF on the day of CIDR removal in a 5-d CO-Synch + CIDR synchronization protocol had excellent timed-AI pregnancy rates that were greater than in cows receiving a single treatment with either PGF or CLP.