Animal Reproduction Science 86 (2005) 37–52 Effects of oestradiol and some of its esters on gonadotrophin release and ovarian follicular dynamics in CIDR-treated beef cattle M.F. Mart´ ınez a , J.P. Kastelic b , G.A. B ´ o c , M. Caccia c , R.J. Mapletoft a,∗ a Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B4 b Research Centre, Agriculture and Agri-Food Canada, Lethbridge, Alberta, Canada T1J 4B1 c Instituto de Reproducci´ on Animal de C ´ ordoba, C´ ordoba 5000, Argentina Received 5 February 2004; received in revised form 28 June 2004; accepted 29 June 2004 Abstract Three experiments were conducted to: (1) compare the effect of three oestradiol formulations on gonadotrophin release in ovariectomised cows; (2) compare the effects of either oestradiol-17 (E- 17) or oestradiol benzoate (EB), given at two doses, on the synchrony of ovarian follicular wave emergence in CIDR-treated beef cattle; and (3) determine the timing of ovulation of the dominant follicle of a synchronised follicular wave following administration of E-17 or EB 24 h after proges- terone withdrawal. In Experiment 1, ovariectomised cows (n = 16) received a once-used CIDR on Day 0 (beginning of the experiment) and were allocated randomly to receive 5 mg of E-17, EB or oestra- diol valerate (EV) plus 100 mg progesterone i.m. The CIDR inserts were removed on Day 7. There were effects of time, and a treatment-by-time interaction (P < 0.0001) for plasma concentrations of both oestradiol and FSH. Plasma oestradiol concentrations peaked 12 h after treatment, with highest (P < 0.01) peak concentrations in cows given E-17; estradiol concentrations subsequently returned to baseline by 36 h in E-17-treated cows and by 96h in EB- and EV-treated cows. Plasma FSH concentrations decreased by 12 h after oestradiol treatment in all groups (P < 0.0001), reached a nadir at 24 h, and increased by 60 h in all groups; plasma FSH reached higher (P < 0.02) concentrations in E-17-treated than in EB- or EV-treated cows. In Experiment 2, non-lactating Hereford cows ∗ Corresponding author. Tel.: +1 306 966 7149; fax: +1 306 966 7159. E-mail address: reuben.mapletoft@usask.ca (R.J. Mapletoft). 0378-4320/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.anireprosci.2004.06.005