Factors affecting the quality and function of the bovine periovulatory follicle

For many cattle operations, profitability depends on the success of reproductive management programs. Opportunities for improving fertility exist within the numerous challenges related to reproductive management. Non-conventional, creative tools for reproductive management could help producers overcome these challenges. In an effort to produce information that could be used to improve reproductive performance of cattle, the following studies were undertaken. The objectives of these studies were threefold: to determine whether GnRH administered as an epidural injection causes ovulation in healthy cows and heifers, to evaluate whether the follicular environment (specifically, follicle fluid) surrounding the oocyte during the maturation phase affects the ability of the cumulus-oocyte complex to progress through early embryonic development, and to investigate the relative effects of estradiol and progesterone on oocyte maturation and early embryo development.

Ability of GnRH to elicit an ovulatory response when administered as an epidural was evaluated in crossbred angus cows and heifers. The preliminary study evaluated this route of administration in crossbred angus cows. Animals were assigned randomly to either intramuscular or epidural administration, and ovaries were visualized via transrectal ultrasound every 6 h until ovulation of the dominant follicle. Results indicated that epidural administration of GnRH was able to trigger an ovulatory response, but timing of ovulation was not measured. The main experiment evaluated incidence of ovulation, time to ovulation, and ovulatory follicle size in crossbred angus heifers administered GnRH either epidurally or intramuscularly. Heifers were randomly assigned to treatment and ovaries were visualized every 4 h via transrectal ultrasound until ovulation of the dominant follicle. Results indicated that epidural administration of GnRH was able to elicit an ovulatory response in heifers, and the timing of ovulation and ovulatory follicle size was not different between administration route. Further investigation is needed to determine if characteristics of the ovulatory response (such as the luteinizing hormone surge) and circulating concentrations of GnRH are altered by epidural administration, which may impact fertility.

GnRH administration is standard practice in many estrous synchronization programs. For fixed-time artificial insemination programs, the detection of estrus prior to insemination has been shown to improve conception and decrease early embryonic loss. The impact of behavioral estrus expression on the oocyte and early embryo were evaluated. Oocytes were matured in vitro in follicle fluid collected from synchronized cows who were classified as having expressed behavioral estrus or not expressing estrus. Embryo cleavage was not affected by estrus expression, but there was a tendency for improved blastocyst development in embryos matured in follicle fluid from animals who had expressed estrus. Cell number was not affected by estrus expression, but future research is needed as to the effect on oocyte acquisition of competence and early embryonic development.

Despite the progress that has been made in culture conditions for in vitro produced embryos, developmental capacity following fertilization is limited at best, with only around one-third of oocytes placed into maturation resulting in viable embryos. During in vivo maturation, the oocyte undergoes final maturation within the follicle, surrounded by a changing microenvironment of estradiol and progesterone. Although the effects of steroids on oocyte development in vitro have been studied on an individual basis, a direct comparison between the ratio of estrogen and progesterone relative to follicle size has not been investigated Effects of steroid hormones estradiol and progesterone on oocyte maturation and early embryonic development were evaluated. Oocytes were matured in vitro in media supplemented with either estradiol, progesterone, or a combination of estradiol and progesterone. Oocytes were fertilized after maturation and cultured for 7 d until development to blastocyst stage. Addition of estradiol alone did not support oocyte maturation or early embryonic development in vitro, and a combination of estradiol and progesterone exhibited an inhibitory effect on oocyte maturation and early embryonic development. Addition of progesterone alone resulted in improved development when compared with estradiol alone or a combination of estradiol and progesterone.

These results indicate that efficiency of reproductive management programs is controlled by multi-faceted factors and opportunities for improvement of reproductive outcomes exist in all of these factors. Although ovulation can be elicited via epidural administration, the impact of this ovulatory trigger on fertility requires further investigation. Display of estrus after synchronization for fixed-time artificial insemination improves conception and decreases early embryonic loss and has a may improve blastocyst development. This effect on early embryo development could be the focus of future research, further improving fertility and possibly the efficacy of in vitro embryo production. Steroid hormones play crucial roles in oocyte competency and the addition of progesterone during in vitro maturation improves development compared with estradiol alone or a combination of estradiol and progesterone.