Fertility in dairy cows following presynchronization and administering twice the luteolytic dose of prostaglandin F _2α as one or two injections in the 5-day timed artificial insemination protocol

The objectives were to evaluate pregnancy per AI (P/AI) of dairy cows subjected to the 5-day timed AI protocol under various synchronization and luteolytic treatments. Cows were either presynchronized or received supplemental progesterone during the synchronization protocol, and received a double luteolytic dose of PGF _2α, either as one or two injections. In Experiment 1, dairy cows (n = 737; Holstein = 250, Jersey = 80, and crossbred = 407) in two seasonal grazing dairy farms were randomly assigned to one of four treatments in a 2 × 2 factorial arrangement. The day of AI was considered study Day 0. Half of the cows were presynchronized (G6G: PGF _2α on Day -16 and GnRH on Day -14) and received the 5-day timed AI protocol using 1 mg of cloprostenol, either as a single injection (G6G-S: GnRH on Day -8, PGF _2α on Day -3, and GnRH + AI on Day 0) or divided into two injections of 0.5 mg each (G6G-T: GnRH on Day -8, PGF _2α on Day -3 and -2, and GnRH + AI on Day 0). The remaining cows were not presynchronized and received a controlled internal drug-release (CIDR) insert containing progesterone from GnRH to the first PGF _2α injection of the 5-day timed AI protocol, and 1 mg of cloprostenol either as a single injection on Day -3 (CIDR-S) or divided into two injections of 0.5 mg each on Days -3 and -2 (CIDR-T). Ovaries were examined by ultrasonography on Days -8 and -3 and plasma progesterone concentrations were determined on Days -3 and 0. In Experiment 2, 655 high-producing Holstein cows had their estrous cycle presynchronized with PGF _2α at 46 ± 3 and 60 ± 3 days postpartum and were randomly assigned to receive 50 mg of dinoprost during the 5-day timed AI protocol, either as a single injection or divided into two injections of 25 mg each. Pregnancies per AI were determined on Days 35 and 64 after AI in both experiments. In Experiment 1, presynchronization with G6G increased the proportion of cows with a CL on Day -8 (80.6 vs. 58.8%), ovulation to the first GnRH of the protocol (64.2 vs. 50.2%), and the presence (95.6 vs. 88.4%) and number (1.79 vs. 1.30) of CL at PGF _2α compared with CIDR cows. Luteolysis was greater for two injections compared to a single PGF _2α injection (two PGF _2α = 95.9 vs. single PGF _2α = 72.2%), especially in presynchronized cows (G6G-T = 96.2 vs. G6G-S = 61.7%). For cows not presynchronized, two PGF _2α injections had no effect on P/AI (CIDR-S = 30.2 vs. CIDR-T = 34.3%), whereas for presynchronized cows, it improved P/AI (G6G-S = 28.7 vs. G6G-T = 45.4%). In Experiment 2, the two-PGF _2α injection increased P/AI on Days 35 (two PGF _2α = 44.5 vs. single PGF _2α = 36.4%) and 64 (two PGF _2α = 40.3% vs. single PGF _2α = 32.6%) after AI. Presynchronization and dividing the dose of PGF _2α (either cloprostenol or dinoprost) into two injections increased P/AI in lactating dairy cows subjected to the 5-day timed AI protocol.