Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: April 4, 2011
Publication Date: July 31, 2011
Citation: Jinks, E.M., Pohler, K.G., Smith, M.F., Macneil, M.D., Roberts, A.J., Waterman, R.C., Geary, T.W. 2011. Effect of ovulatory follicle size and estradiol supplementation during the preovulatory period on pregnancy rates in postpartum beef cows. 44th Annual Meeting of the Society for the Study of Reproduction. Reproduction and the World's Future. Abstract #479. Portland, Oregon. Technical Abstract: In postpartum beef cows, GnRH-induced ovulation of small dominant follicles decreased pregnancy rates and increased late embryonic/fetal mortality; however, ovulatory follicle size had no apparent effect on the establishment or maintenance of pregnancy when ovulation occurred spontaneously. Furthermore, small ovulatory follicle size was associated with reduced circulating preovulatory concentrations of estradiol and lower pregnancy rates. Preovulatory estradiol aids in providing a maternal environment that is conducive to the establishment of pregnancy, including induction of endometrial progesterone receptors, decreased uterine pH at estrus, and preparation of granulosa cells for luteinization. Our hypothesis was that supplemental estradiol would increase the probability of pregnancy in postpartum beef cows induced to ovulate a small dominant follicle. Objectives of the first experiment were to examine effects of estradiol supplementation on preovulatory concentrations of estradiol and postovulatory concentrations of progesterone in serum of postpartum beef cows. Suckled beef cows (n = 42) were treated with the CO-Synch + CIDR protocol (GnRH and insertion of CIDR on d -9, PGF2a and CIDR removal d -2, and GnRH plus artificial insemination [AI] d 0). On d -1, cows received an injection of estradiol cypionate (ECP; 0.5 mg, i.m) or sham injection. Serum concentrations of estradiol (d -1 to d 3) and progesterone (d 0 to d 14) were similar between treatments (P = 0.74 and 0.33, respectively). Proportion of cows expressing estrus was 5/16 and 8/13 for ECP and Control, respectively (P = 0.10). A second experiment was conducted, over three years, to determine effect of ovulatory follicle size and estradiol supplementation during the preovulatory period on establishment of pregnancy in postpartum beef cows. As in experiment 1, cows (n = 602) were treated with the CO-Synch + CIDR protocol and approximately half of the cows received ECP (0.5 mg, i.m) on d -1. Transrectal ultrasonography was used to measure follicle size at time of AI (d 0) and to determine pregnancy status and fetal viability (presence of a heartbeat) on d 28 following AI. There was a significant effect of year on pregnancy rate (P < 0.01), but interactions between year and treatment and/or follicle size did not affect pregnancy rate. Logistic regression of pregnancy rate on follicle size were curvilinear and significantly different (P < 0.01) for ECP and Control treatments. Compared with the Control, ECP treatment increased pregnancy rates of cows induced to ovulate small dominant follicles. In summary, estradiol supplementation during the preovulatory period had no effect on circulating estradiol or postovulatory circulating concentrations of progesterone but increased pregnancy rates in postpartum beef cows that were induced to ovulate a small dominant follicle. Research supported by NRI grant 2006-35203-17284 from USDA-CSREES.