The objective was to evaluate the effects of GnRH or PGF(2 alpha)-based synchronization and resynchronization programs on fertility in lactating dairy cows. For experiment 1, cows (n = 1,521) were presynchronized with 2 injections of PGF given at 36 and 50 DIM and assigned to 1 of 3 protocols: Ovsynch [OVS; n = 552; GnRH injection, PGF(2 alpha), injection 7 d later, GnRH injection 56 h later, and timed artificial insemination (TAI) 16 h later] beginning at 14 d after presynchronization (PS), GnRH-GnRH-PGF(2 alpha)-GnRH (GGPG; n = 402) treatment with a GnRH injection given 7 d after PS and OVS 7 d later, or PGF(2 alpha)-GnRH-PGF(2 alpha)-GnRH (P7GPG; n = 567) treatment with a PGF(2 alpha) injection given 7 d after PS and OVS beginning 7 d later. Experiment 2 cows (n = 2,327) were assigned to 1 of 3 resynchronization protocols 7 d before nonpregnancy diagnosis (NPD): GGPG cows (n = 458) received a GnRH injection at enrollment and OVS at NPD; P7GPG cows (n = 940) received a PGF(2 alpha), injection at NPD and OVS 7 d later; and P11GPG cows (n = 929) received a PGF(2 alpha), injection 3 d after NPD and OVS 11 d later. In both experiments, cows were artificially inseminated upon estrus detection (ED). In experiment 1, 52.3% of cows were artificially inseminated upon ED, with GGPG having reduced ED (GGPG = 46.8 vs. OVS = 50.7 and P7GPG = 57.7%). Treatments did not affect overall pregnancy per artificial insemination (P/AI) at 36 and 66 d after AT (OVS = 34.1 and 32.3, P7GPG = 34.6 and 31.9, and GGPG = 31.3 and 28.1%, respectively) or pregnancy loss but cows artificially inseminated upon ED had higher P/AI than cows undergoing TAI (ED cows = 37.9 vs. TAI cows = 28.8%). Treatment did not affect P/AI for cows artificially inseminated upon ED or TAI at 36 and 66 d after AT (OVS = 34.1 and 32.3, P7GPG = 34.6 and 31.9, and GGPG = 31.3 and 28.1%). Median days in milk at first AT was affected by treatment (P7GPG = 59 vs. OVS = 68 and GGPG = 68 d). In experiment 2, GGPG reduced ED (GGPG = 23.3 vs. P7GPG = 74.9 and P11GPG = 79.6%). Treatment did not affect overall P/AI at 36 and 66 d after AT (GGPG = 29.2 and 25.8, P7GPG = 28.7 and 26.6, and P11GPG = 31.9 and 30.2%) or pregnancy loss. Cows artificially inseminated upon ED had greater P/AI than TAI cows (ED = 32.3 and TAI = 25.1%). However, treatment did not affect P/AI for cows artificially inseminated upon ED at 36 and 66 d after AT (GGPG = 29.6 and 27.3, P7GPG = 29.4 and 28.1, and P11GPG = 35.7 and 33.7%) or TAI (GGPG = 29.1 and 25.3, P7GPG = 25.0 and 22.1, and P11GPG = 16.9 and 16.9%). Median days between NPD and AT was affected by treatment (GGPG = 10 vs. P7GPG = 4 and P11GPG = 7 d). Prostaglandin-based programs increased ED and reduced interval to first AT and between AI. Gonadotropin-releasing hormone-based programs increased the proportion of TAI cows. Cows artificially inseminated upon ED had increased P/AI compared with TAI cows.
