Work

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

Year: 2026

Type: article

Abstract: 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.

Source: Journal of Dairy Science

DOI: 10.3168/jds.2025-27334