Increasing gonadorelin hydrochloride dose at the first GnRH of breeding Ovsynch enhances luteinizing hormone release, ovulatory response, and pregnancy in first-service lactating dairy cows

The objective of this study was to evaluate the effect of administering 200 µg versus 100 µg of GnRH as gonadorelin hydrochloride (Factrel; Zoetis, Parsippany, NJ) at the first GnRH injection (G1) of the breeding Ovsynch in a Double-Ovsynch (DO) protocol on pregnancy per artificial insemination (P/AI) at first service in lactating dairy cows. The study was conducted across 4 commercial dairies located in California (n = 2) and Wisconsin (n = 2). We also assessed the effect of GnRH dose on ovulatory response and LH release. Each week, cows were blocked in a 2 × 2 factorial design by parity (primiparous or multiparous) and GnRH dose (100 µg or 200 µg) within each herd. The primary outcome was P/AI following estrus synchronization and timed AI. Cows were excluded if they were inseminated outside the synchronization window, culled before pregnancy diagnosis, or missed scheduled GnRH or PGF2α treatments. Dinoprost tromethamine (12.5 mg/mL; Lutalyse HighCon, Zoetis) served as the PGF2α source. A total of 6,621 cows met the inclusion criteria (2,525 primiparous; 4,096 multiparous). Of these, 3,310 received 100 µg GnRH (2 mL), and 3,311 received 200 µg GnRH (4 mL). A subset of 505 cows from herd A underwent transrectal ultrasonography to assess ovulatory response at G1 and 7 d later (first PGF2α of DO), with concurrent serum progesterone (P4) sampling. An additional subset of 18 cows had blood samples collected at 0 (pretreatment) and at 30, 60, 90, 120, 150, 180, 210, 240, 300, and 360 min post-treatment to measure LH concentrations. The final statistical model for P/AI included fixed effects of treatment, parity, and their interaction, with random effects of herd, service sire, herd-by-treatment-by-parity, and month fresh nested within herd-by-parity. Treatment and parity significantly affected P/AI, although their interaction was not significant. Least squares means for P/AI were higher with 200 µg than 100 µg GnRH (51.8% vs. 48.6%). When stratified, the effect in primiparous cows was minimal (52.8% vs. 51.8%), but substantial in multiparous cows (50.7% vs. 45.4%). Cows treated with 200 µg GnRH had a greater ovulatory response than those receiving 100 µg (83.0% vs. 70.8%). Parity also influenced ovulation, though no treatment-by-parity interaction was observed. When P4 was categorized (Low <3.0 ng/mL, High ≥3.0 ng/mL), a 3-way interaction emerged: in primiparous cows with Low-P4, ovulation was greater with 200 µg; in multiparous cows with High-P4, ovulation was greater with 200 µg. No treatment effect was observed on P4 concentrations 7 d post-treatment. Cows receiving 200 µg GnRH had higher peak LH and area under the curve than those receiving 100 µg, though peak timing did not differ. In summary, increasing the GnRH dose to 200 µg at G1 of the breeding Ovsynch in a DO program improved LH release, ovulatory response, and first-service P/AI, with the greatest benefit in multiparous cows.