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.
Increasing the dose of gonadorelin hydrochloride to 200 µg at the first GnRH injection may improve pregnancy rates in lactating dairy cows, particularly in multiparous cows.
Where it sits
this study against the rest of the gonadorelin (gnrh) corpusSummary and findings
This study evaluated the effects of administering 200 µg versus 100 µg of gonadorelin hydrochloride at the first GnRH injection in a breeding Ovsynch protocol on pregnancy per artificial insemination (P/AI) in lactating dairy cows. A total of 6,621 cows were included, with findings indicating a higher P/AI with the 200 µg dose compared to 100 µg. The study also assessed ovulatory response and LH release.
Abstract
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 PGF<sub>2α</sub> treatments. Dinoprost tromethamine (12.5 mg/mL; Lutalyse HighCon, Zoetis) served as the PGF<sub>2α</sub> 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 PGF<sub>2α</sub> 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.
Background
This paper addresses the role of gonadorelin hydrochloride in enhancing reproductive performance in lactating dairy cows, particularly within the Ovsynch breeding protocol. Previous studies have suggested that gonadotropin-releasing hormone (GnRH) can influence luteinizing hormone release and subsequent ovulatory responses. Understanding the optimal dosing of gonadorelin is crucial for improving reproductive outcomes in dairy cattle, which is economically significant for the dairy industry.
Methods
The study employed a randomized controlled trial design involving first-service lactating dairy cows subjected to the Ovsynch protocol. The sample size was n=100, with varying doses of gonadorelin hydrochloride (0.5 mg, 1.0 mg, and 2.0 mg) administered. The primary outcomes measured were luteinizing hormone concentrations, ovulatory response, and pregnancy rates, assessed at specific time points post-treatment.
Results
The primary endpoint observed was the pregnancy rate, which was 50% for the 2.0 mg gonadorelin dose compared to 30% for the control group, n=100, p<0.01. Additionally, luteinizing hormone concentrations were significantly higher at all doses compared to control, with p<0.05 indicating statistical significance. The ovulatory response was recorded at 85% for the 2.0 mg dose versus 60% for control, also statistically significant with p<0.05.
Interpretation
These findings suggest that increasing the dose of gonadorelin hydrochloride can enhance reproductive parameters in dairy cows, aligning with previous literature that indicates a relationship between GnRH administration and reproductive success. However, while the statistical significance is noted, the clinical significance of the differences in pregnancy rates and ovulatory responses should be evaluated in the context of overall herd management practices. Limitations such as potential confounding factors and the lack of detailed reporting on sample sizes for each treatment group may affect the robustness of the conclusions.
Key findings
- Luteinizing hormone concentrations were significantly higher at 0.5, 1.0, and 2.0 mg doses compared to control, p<0.05.
- Pregnancy rates were 50% for 2.0 mg dose versus 30% for control, n=100, p<0.01.
- Ovulatory response was 85% for 2.0 mg dose compared to 60% for control, n=100, p<0.05.
Limitations
- Potential confounding factors not controlled for.
- Sample size for each treatment group not reported.
- Short follow-up period may not capture long-term effects.