The objectives of this study were to develop short oestrus synchronization methods allowing timed artificial insemination (TAI) in lactating dairy cows and to compare the synchronizing effects of the use of human chorionic gonadotropin (hCG), instead of a gonadotropin releasing hormone (GnRH), on ovulation and pregnancy rates. An additional goal was to determine the effects of the presence or absence of an active corpus luteum (CL) on the treatment efficacy at the beginning of treatment. Sixty-three cows at random stages of the oestrus cycle on the 50th-95th postpartum day were randomly distributed into two groups. Cows of GnRH group (n = 33) received prostaglandin F-two alpha (PGF, 0.150 mg), estradiol propionate (EP, 2 mg) and GnRH (50 µg) in 24 hour intervals (PGF at hour 0; EP at hour 24 and GnRH at hour 48). Cows in the hCG group (n = 30) were treated in a similar manner to the GnRH group, but, alternately, these cows received hCG (500 IU, i.m.) instead of GnRH. All cows in the treatment group were inseminated timely 16-20 hours after GnRH or hCG injections regardless of oestrus signs. During the study, animals exhibiting natural oestrus were inseminated 10-12 hours later and served as controls (n = 44). Cows in the hCG group have significantly higher synchronized (P = 0.018) and total (P < 0.05) ovulation rates and shorter intervals between the last hormone injection and ovulation (P = 0.012) compared to cows in the GnRH group. In both GnRH and hCG groups, cows with an active CL at the beginning of the treatment have acceptable pregnancy rates (40.0% and 47.6% respectively, P > 0.05). The PGF/EP/GnRH and PGF/EP/hCG treatments resulted in comparable pregnancy rates after TAI of lactating dairy cows at random stages of the oestrus cycle relating to those inseminated at natural oestrus (33.3%, 40.0% and 40.9% respectively, P > 0.05).
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