Effect of ovulatory follicle size on steroidogenic capacity, molecular markers of oocyte competence and bovine pregnancy associated with glycoproteins in beef cows

Abstract

Artificial insemination (AI) and estrus synchronization (ES) are effective tools for the genetic advancement of cattle. Gonadotropin-releasing hormone (GnRH) is used at the beginning of ES protocols to induce ovulation and start a new follicular wave, and(or) at the end of a protocol to induce ovulation in combination with insemination. However, GnRH-induced ovulation of physiologically immature follicles negatively affected pregnancy rates in beef heifers and postpartum cows and increased late embryonic/early fetal loss. Mechanisms associated with reduced pregnancy rates and late embryonic/fetal survival are not clear, but may be due to inadequate oocyte competence and(or) a compromised uterine environment. Therefore, two studies were conducted to deterimine the effect of ovulatory follicle size on oocyte competence (experiment 1) and placental function (experiment 2-4). Based on the above experiments, ovulatory follicle diameter was positively associated with serum and intrafollicular estradiol; however, the relationship between ovulatory follicle size and markers of oocyte competence was not conclusive (experiment 1). Furthermore, there was no relationship between bovine pregnancy associated glycoproteins (bPAGs; marker of placental function) and ovulatory follicle size, stage of embryonic development, or embryo quality; however, cows that lost an embryo after d 28 of gestation had lower concentrations of bPAGs on d 28 compared to cows that maintained pregnancy (experiment 2-4).