The repeatability of gonadotropin releasing hormone-induced release of luteinizing hormone and its association with fertility in dairy cattle

Abstract Text:

Objectives were to: (1) determine repeatability and variability of plasma LH concentrations in response to exogenous GnRH administration and (2) examine associations among categories of LH release, plasma estradiol, ovulatory response and first service conception rate in dairy cattle. Lactating Holstein cows (35 primiparous, 65 multiparous) received one injection of PGF₂α (cloprostenol, 500 µg, d 0) followed by GnRH (gonadorelin, 100 µg, d 3; Presynch) and were subjected to an Ovsynch protocol starting on d 10, with timed-AI (TAI) occurring at ~75 d postpartum. Blood samples were collected immediately before (0 h) and 2 h after the GnRH of Presynch and the second GnRH of Ovsynch to determine plasma LH concentrations. Cows were ranked based on LH concentrations after the second GnRH of Ovsynch, from highest to lowest, and those in the top (n=33) and bottom (n=33) thirds were classified into HIGH- and LOW-LH categories. Differences in plasma LH and estradiol concentrations among parity and LH categories were analyzed using MIXED procedure of SAS. Repeatability was analyzed using the CORR procedure and binomial data using the GLIMMIX procedure of SAS. Mean (±SEM) LH concentrations (ng/mL) before GnRH were 0.4±0.04 and 0.6±0.03, while, the mean LH 2 h after GnRH were 8.3±0.7 (range 1.0 to 27.4, CV 80.7%) and 10.0±0.7 (range 0.7 to 28.4, CV 67.3%), during Presynch and Ovsynch assessments, respectively. The correlation between GnRH-induced LH concentrations during Presynch and Ovsynch assessments was r=0.19 (P=0.06). The proportion of cows that remained in HIGH- and LOW-LH categories during both Presynch and Ovsynch assessments was 35.3 and 33.3 %, respectively. The mean plasma LH concentration (ng/mL) after GnRH was significantly greater (P<0.01) for HIGH-LH (17.6±0.6) than LOW-LH (2.8±0.6) category. Similarly, cows in the HIGH-LH category had greater plasma estradiol than those in LOW-LH category (2.7±0.3 vs. 1.1±0.3 pg/mL; P<0.01). In addition, cows in the HIGH-LH category had greater ovulatory response (97.0 vs. 78.8 %; P=0.03) and increased first service conception rate (44.1 vs. 24.2 %; P=0.04) than cows in the LOW-LH category. The mean plasma LH concentration in multiparous cows was significantly greater at Presynch assessment (9.3±0.8 vs. 6.3±1.1; P=0.03) and numerically greater at Ovsynch assessment (10.7±0.8 vs. 8.7±1.1; P=0.16) than in primiparous cows. In summary, GnRH-induced LH concentrations were highly variable and weakly repeatable. However, cows with higher GnRH-induced plasma LH concentrations 2h after second GnRH of Ovsynch had greater ovulatory response and conception rates.

Keywords: LH variability, repeatability, fertility