Pre-synchronization with GnRH or hCG enhances fertility outcomes of heat synch protocol in Holstein dairy cows across parities: A pilot study

Efficient estrus synchronization is important for improving reproductive performance in dairy herds; however, variations in ovarian activity among cows of different parities may influence responses to synchronization protocols. The present study evaluated the effects of pre-synchronization with gonadotropin-releasing hormone (GnRH) or human chorionic gonadotropin (hCG) before the Heat Synch protocol on estrus expression, pregnancy outcomes, and serum progesterone concentrations in Holstein cows of different parities. The Heat Synch protocol consisted of GnRH administration on Day 0, prostaglandin F₂α (PGF₂α) administration on Days 7 and 8, and estradiol benzoate administration on Day 9 before insemination. A total of 294 Holstein cows were allocated to nine groups: Heat Synch (HS) alone, GnRH+HS, or hCG+HS in primiparous (n = 99), second-parity (n = 99), and multiparous (n = 96) cows. Estrus detection was followed by artificial insemination according to the AM–PM rule, whereas cows that failed to exhibit estrus underwent fixed-time artificial insemination. Pregnancy was diagnosed by ultrasonography on Days 30 and 60 post-insemination. Serum progesterone concentrations were evaluated at the beginning of the study, before PGF₂α administration, and before estradiol benzoate administration using ELISA. Pre-synchronization with GnRH or hCG significantly increased estrus expression compared with the HS protocol alone. The hCG groups showed higher progesterone concentrations, and no statistically significant parity-related differences in reproductive responses to the synchronization protocols were detected under the conditions of this study. These findings suggest that GnRH- or hCG-based pre-synchronization before Heat Synch may improve reproductive performance under practical dairy farm conditions while promoting behavioral estrus expression before insemination. However, the relatively limited sample size within some subgroups should be considered when interpreting parity-related comparisons.