Replacing the first gonadotropin-releasing hormone treatment in an Ovsynch protocol with human chorionic gonadotropin decreased pregnancies per artificial insemination in lactating dairy cows

Our objective was to compare the effect of treatment with GnRH at first (G1) Breeding-Ovsynch portion a Double-Ovsynch (DO) protocol human chorionic gonadotropin (hCG) on pregnancies per artificial insemination (P/AI) in lactating dairy cows. In experiment 1, cows (n = 1,932) submitted DO for timed (TAI) 2 commercial farms were blocked by parity (primiparous vs. multiparous) and randomly assigned receive 100 µg versus 2,500 IU hCG G1. Overall, P/AI 39 d after TAI inseminated sexed semen greater treated than within each (primiparous: 42.6% 38.2%; multiparous: 39.4% 30.3%). Similarly, multiparous conventional beef tended be (41.1% 34.3%). 2, Holstein 43) protocols described 1. Ovaries evaluated transrectal ultrasonography immediately before 24, 28, 32, 36, 40 h assess time from ovulation, blood samples collected G1, PGF2α treatment, 8 16 later, second (G2) luteolysis based serum progesterone (P4) concentrations. Although mean (± standard error mean) ovulation approximately (33.7 ± 0.6 31.5 h), P4 concentrations during proportion complete (P4 <0.4 ng/mL G2) did not differ between treatments. We conclude that replacing G1 decreased fertility but affect rate or completeness despite increased interval ovulation.