Submitted to: Journal of Animal Science Supplement
Publication Type: Abstract only
Publication Acceptance Date: 5/15/2006
Publication Date: 7/15/2006
Citation: Atkins, J.A., Geary, T.W., Wells, K.J., Lucy, M.C., Smith, M.F. 2006. Factors affecting ovulatory follicle size and ovulation success to gnrh-induced ovulation in postpartum beef cows. Journal of Animal Science Supplement Abstract #210. Interpretive Summary: Among estrous cycling, lactating beef cows, day of the estrous cycle at the onset of ovulation synchronization with the CO-Synch protocol affects ovulatory response to the first injection of GnRH. However, neither day of the estrous cycle at the start of synchronization nor ovulatory response to the first injection of GnRH affected the ovulatory response to the second GnRH injection. Cows that exhibited estrus before or at the time of the second injection of GnRH and cows ovulating larger follicles in response to the second GnRH injection developed corpora lutea that produced progesterone at a greater rate than cows not exhibiting estrus and cows ovulating smaller follicles. This increased rate of progesterone secretion may improve a developing embryo’s ability to establish pregnancy.
Technical Abstract: In the present study, the effects of day of the estrous cycle and ovulatory response at the first GnRH injection on size of the dominant follicle at the second GnRH were examined in multi-parous lactating beef cows (n=60). GnRH was administered on day -9 (GnRH1), prostaglandin F2' on day -2, and GnRH on day 0 (GnRH2). Cows were classified into groups based on day of the estrous cycle (Day 2, 5, 9, 13, or 18; Day 0 = estrus) at GnRH1. With the exception of cows in the Day 2 group (0%), the proportion ovulating to GnRH1 (63%) did not differ (P >0.10) among treatment groups. Neither day of the estrous cycle nor ovulatory response at GnRH1 affected (P > 0.10) size of the dominant follicle or proportion ovulating at GnRH2. Cows that ovulated a follicle ' 13 mm had a higher rate of increase in progesterone (P4) following ovulation (d2-d12; d0=GnRH2) than cows that ovulated 11-12mm (P<0.05) follicles or ' 10mm follicles (P<0.01). Cows that displayed estrus at GnRH2 had a higher rate of increase in P4 (d2-12) than cows that did not exhibit estrus (P<0.001). In summary, ovulation at GnRH1 did not affect follicle size or proportion ovulating at GnRH2. In cows that ovulated at GnRH2, those that ovulated a follicle ' 13 mm and(or) expressed estrus had an increased rise in P4 compared to cows that ovulated smaller follicles or did not display estrous behavior.