Effect of preovulatory follicle maturity on pregnancy establishment in cattle: the role of oocyte competence and the maternal environment

ABSTRACT

Reproductive technologies to synchronize estrus and ovulation in cattle have enhanced the ability to practically utilize artificial insemination to increase both genetic merit and reproductive management of beef and dairy herds. The ability to successfully synchronize a follicular wave and ovulation, in heifers and cows, has improved substantially in recent years. Consequently, pregnancy rates to a single fixed-time artificial insemination (FTAI) can approximate that of insemination following spontaneous estrus. Despite these advances, a subset of heifers and cows often has a physiologically immature dominant follicle at the time of GnRH-induced ovulation. These animals will exhibit reduced pregnancy rates and decreased embryonic survival if a pregnancy happens to become established. The physiological mechanisms underlying the preceding decreased fertility have been a focus of our laboratories and may include an effect of the follicular microenvironment on both oocyte competence and the maternal environment. Oocytes must have adequate opportunity to complete cytoplasmic and molecular maturation during the final stages of oocyte maturation that occur within the preovulatory follicle. Follicular status, during the proestrus period, must be such that adequate circulating concentrations of estradiol are present before FTAI to increase oviductal transport of gametes and enhance both the luteinizing capacity of granulosa cells and progesterone receptor population in the post-ovulatory uterus. Following ovulation, the follicle’s transformation to a functional corpus luteum to secrete adequate amounts of progesterone is essential for the establishment of pregnancy. The physiological status of the preovulatory follicle, prior to FTAI, greatly affects the concepts discussed above and has an important impact on pregnancy establishment and maintenance in cattle.