Uterine health and reproductive hormones in genetically divergent Holstein-Friesian and Jersey cows evaluated under seasonal pasture-based management systems

The objective was to identify the physiological mechanisms underpinning differences in fertility in Holstein-Friesian and Jersey (J) cow populations with divergent genetic backgrounds. The study was conducted over 2 yr using animals comprising 2 divergent genetic groups (GG) of Holstein-Friesian cows: elite and national average (NA). The elite cows were representative of the top 5% of cows in the Irish national herd for the Economic Breeding Index (EBI), and the NA cows were representative of the national average EBI cow in Ireland. In addition, a single GG of purebred J cows was included that was representative of J genetics available in Ireland and used primarily for crossbreeding. Cows in each GG were randomly assigned to 1 of 3 spring-calving pasture-based feeding treatments (FT) that commenced immediately after calving: control (CTL), low grass allowance (LGA), and high concentrate (HC). After parturition, vaginal discharge scores were recorded twice weekly across 2 yr on elite (n = 134), NA (n = 73), and J (n = 76) cows until discharge was free of mucopurulent material (score ≤1; i.e., either no discharge or clear mucus). The estrous cycles of 20 elite, 20 NA, and 20 J cows were synchronized to facilitate determination of plasma estradiol (E2) and progesterone (P4) concentrations and measurement of ovarian structures using transrectal ultrasonography. The interval from calving to a vaginal discharge score ≤1 was shorter in (1) Elite versus NA or J cows; (2) cows in parities 2 and 3+ versus parity 1 cows; (3) cows that had delivered calves with medium birth weight versus cows that delivered calves with light or heavy birth weights; and (4) cows with a normal calving compared with cows that had a difficult calving. No effect of GG was found on plasma E2 concentrations, preovulatory follicle diameter, or postovulatory corpus luteum volume. The elite cows had greater mean (95% CI) luteal phase plasma P4 concentrations compared with NA and J cows: 7.3 (6.4, 8.3), 5.2 (4.2, 6.4), and 6.0 ng/mL (5.1, 7.05), respectively. Mean preovulatory plasma E2 concentrations were greater in cows assigned to the LGA and CTL FT compared with cows assigned to HC: 3.6 (2.6, 4.7), 3.1 (2.5, 3.8), and 2.1 pg/mL (1.5, 2.8), respectively. Likewise, cows assigned to CTL and LGA FT had greater mean plasma P4 concentrations than cows assigned to HC FT: 6.5 (5.6, 7.4), 7.2 (5.9, 8.7), and 5.2 ng/mL (4.3, 6.2), respectively. Improved postpartum reproductive tract health status and greater luteal phase plasma P4 concentrations are 2 potential physiological mechanisms that underpin differences in fertility performance between GG. These mechanisms contribute to a more favorable environment for the establishment and maintenance of pregnancy and may explain how genetic selection for fertility traits can improve phenotypic cow fertility. The results also highlight the potential role for cow nutritional management in pasture-based systems to influence the reproductive axis and, in some cases (high-concentrate feeding), counteract the biological response to genetic selection.