Effect of using 200 μg of gonadorelin at the first gonadotropin-releasing hormone of the breeding-Ovsynch on ovulatory response and pregnancies per artificial insemination in first-service lactating Holstein cows

This study aimed to determine whether 200 μg of GnRH (gonadorelin hydrochloride) would increase ovulatory response and pregnancies per artificial insemination (P/AI) compared with 100 μg at the first GnRH of the breeding-Ovsynch of a Double-Ovsynch program (DO) in lactating Holstein cows. Weekly cohorts of primiparous (n = 719) and multiparous (n = 1,191) cows submitted to DO (GnRH, 7 d later PGF , 3 d later GnRH, 7 d later GnRH [G1], 7 d later PGF [PG1], 1 d later PGF , ∼32 h later GnRH [G2], and ∼16 h later timed artificial insemination [TAI]) for first service, randomly received either 100 μg or 200 μg of GnRH (gonadorelin hydrochloride) at G1 (primiparous, 64-75 DIM; multiparous, 59-70 DIM). Ovulation was determined by ultrasound 2 d after G1 (n = 1,294) and 2 d after G2 (n = 1,020). Blood samples were collected at G1 and at PG1 d to evaluate serum progesterone (P4) concentrations. Conventional (n = 314, Angus; n = 1,084, Holstein) and Holstein sexed semen (n = 276) were used. Pregnancy was diagnosed on d 32, 46, 88, and 200 post-TAI. The high dose of GnRH (200 μg) increased overall ovulatory response to G1 compared with 100 μg (81.3% vs. 65.1%), being similar between parities (primiparous, 72.2%; multiparous, 73.9%). Mean serum P4 concentrations at PG1 did not differ between treatments (100 µg: 9.59 ± 0.15 ng/mL vs. 200 µg: 9.43 ± 0.15 ng/mL). Cows with no ovulation to G1 had higher serum P4 concentrations at G1 than cows with ovulation to G1 (6.27 ± 0.19 ng/mL vs. 4.66 ± 0.07 ng/mL). At PG1, the proportion of cows with functional corpus luteum (98.7% vs. 89.7%) and serum P4 concentrations (9.68 ± 0.12 ng/mL vs. 9.14 ± 0.22 ng/mL) were greater in cows that ovulated to G1 compared with cows that did not ovulate. Also, cows that ovulated to G1 had a greater increase in serum P4 concentrations from G1 to PG1 than cows with no ovulation (5.26 ± 0.12 ng/mL vs. 3.32 ± 0.25 ng/mL). The high dose of GnRH improved overall P/AI at 32 d post-TAI in cows inseminated with conventional semen (54.6% vs. 48.2%) and tended to improve P/AI on 46 (48.8% vs. 44.9%), 88 (47.6% vs. 43.4%), and 200 (45.3% vs. 41.2%) d post-TAI. Primiparous cows inseminated with conventional semen had better P/AI than multiparous cows at d 32 (58.2% vs. 49.4%), 46 (55.1% vs. 44.4%), 88 (53.2% vs. 43.2%) and 200 (51.6% vs. 40.7%) post-TAI. Primiparous cows treated with 200 µg GnRH had lower P/AI on d 32, 46, 88, and 200 post-TAI when inseminated with sexed semen than with conventional semen.