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Veterinary Medicine Teaching and Research Center, University of California-Davis, Tulare 93274
Corresponding author: J. E. P. Santos; e-mail: Jsantos{at}vmtrc.ucdavis.edu.
A total of 799 Holstein cows from 3 herds were randomly assigned at 37 ± 3 d in milk (DIM) to timed artificial insemination (AI) or insemination at detected estrus. Cows were presynchronized with injections of PGF2
at 37 and 51 DIM. At 65 DIM, cows received an injection of GnRH, followed 7 d later by PGF2. Cows in the estrus-detected group were inseminated after being observed in estrus during the 7 d after the last PGF2. Cows in the timed AI group received an injection of 1 mg of estradiol cypionate (ECP) 24 h after the last PGF2. If detected in estrus 
24 h after ECP, cows were inseminated then or at a fixed time 48 h after ECP. Pregnancy was diagnosed by ultrasonography at 30 d and by palpation at 44 and 58 d after AI. Plasma progesterone was measured in 4 samples per cow collected on days of 1) second PGF2, 2) GnRH, 3) third PGF2, and 4) 48 h after third PGF2. Cows were classified as cyclic or anovulatory based on progesterone concentrations in samples 1 and 2. Similarly, cows were classified according to progesterone concentrations in samples 2, 3, and 4 (H = 
1 ng/mL; L = <1 ng/mL), resulting in 8 combinations (LLL, LHL, LLH, LHH, HHH, HHL, HLH, and HLL). Conception rates and pregnancy rates were higher for cows in the timed AI group than in the estrus-detected group at 30, 44, and 58 d (e.g., at 58 d, pregnancy rates were 42.2% for multiparous cows or 34.4% for primiparous cows in the group receiving ECP and timed AI compared with only 20.8 or 18.8% for respective parity subgroups for the treatment group inseminated only at detected estrus). Pregnancy losses were 11.5% from 30 to 58 d and did not differ between treatments. Cyclic cows within both treatments had higher estrous responses, conception rates, and pregnancy rates. Cows that responded to presynchronization and to luteolysis (HHL) had the highest conception and pregnancy rates, followed by cows classified as LHL. Use of 1 mg of ECP to induce ovulation as part of a synchrony regimen improved reproduction at first postpartum insemination in dairy cows.
Key Words: timed artificial insemination • estradiol cypionate • dairy cow • reproduction
Abbreviation key: AOR = adjusted odds ratio, CI = confidence interval, CL = corpus luteum, ECP = estradiol cypionate.
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