Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2012
Publication Date: 11/12/2012
Publication URL: http://handle.nal.usda.gov/10113/56114
Citation: Atkins, J.A., Smith, M.F., MacNeil, M.D., Jinks, E.M., Abreu, F.M., Alexander, L.J., Geary, T.W. 2012. Pregnancy establishment and maintenance in cattle. Journal of Animal Science. 91:722-733. Interpretive Summary: Cows that ovulate a smaller follicle after an injection of gonadotrophin releasing hormone have a reduced chance of becoming pregnant. This results from both reduced pregnancy establishment and greater embryonic death. We assumed ovulation of an immature ovum or from an immature follicle to be the root cause. To determine the root cause, we conducted a reciprocal embryo transfer study. Cows were identified as ovulating either a large (>12.5 mm) or a small (< 12.5 mm) follicle on day 0. Embryo transfers were planned so that follicle sizes of donor and recipient cows were independent. Single embryos (n = 394) were recovered from donor cows 7 days after breeding and all live embryos were transferred into recipient cows. In the donor cow, greater body weight, circulating estradiol (E2) concentration on day 0, days since last calving, and follicle size increased fertilization success. Only age of the donor cow had a negative influence on fertilization success. Thus, younger cows had greater fertilization success. More rapid follicular growth from day -2 to 0 and increased body weight decreased the percentage of viable embryos on day 7. Thus, while heavier cows experienced greater fertilization success, they provided more embryos that died. Increased serum progesterone (P4) concentration on day -2 and ovulation of larger follicles by donor cows increased embryo survival to day 7. In the recipient cow, embryo survival from day 7 to 27 was enhanced by increased serum E2 concentration on day 0 and P4 concentration on day 7. Both of these hormones help prepare the uterus for a successful pregnancy. Increased ovulatory follicle diameter in the recipient cow on day 0 increased embryonic loss from day 7 to 27. We interpret this to mean that the corpus luteum resulting from ovulation of a large follicle may be inherently shorter lived allowing less time for the embryo to signal its presence in the uterus. Pregnancy success depends on the complex interplay of hormonal and physical factors before and after fertilization that influence both oocyte competence and maternal environment. Increased E2 and subsequent P4 concentrations are direct effects of a larger ovulatory follicle. Thus, increased fertility in cows ovulating larger follicles may be mediated by increased production of E2 and P4 and their effects on the uterus.
Technical Abstract: GnRH-induced ovulation of a small dominant follicle reduced pregnancy success in cattle. A reciprocal embryo transfer study was conducted at Fort Keogh from 2007 to 2009 in order to differentiate between follicular effects on pregnancy mediated through oocyte quality or uterine environment. Estrous cycles were synchronized in suckled beef cows and embryo donors were inseminated on day 0 (n = 810). Embryos (n = 393) or oocytes (n = 44) were recovered on day 7 and all viable embryos were transferred into recipients (n = 354) the same day. Both donors and recipients were classified as ovulating either a small (<12.5 mm) or large (=12.5 mm) diameter follicle and transfers occurred in a 2x2 factorial arrangement of these categories. Factors with direct influence (positive, except age) on fertilization of the structure recovered on day 7 (in order of importance) were donor cow weight, age, circulating estradiol concentration on day 0, days postpartum, and ovulatory follicle size. Viability of day 7 embryos was directly inhibited by rapid follicular growth rate from day -2 to 0 and increased body weight. Direct beneficial effects to embryo viability were increased serum progesterone concentration on day -2 and ovulatory follicle size. Pregnancy maintenance from day 7 to 27 was enhanced by estradiol concentration on day 0 and increased serum progesterone concentration on day 7 in the recipient cow. Increased follicle diameter in the recipient cow on day 0 was detrimental to pregnancy maintenance from day 7 to 27. Establishment and maintenance of pregnancy depends on the complex interplay of endocrine and physical factors both prior and subsequent to fertilization and influencing both oocyte competence and maternal environment.