Animal Reproduction Science 67 (2001) 221–229 The use of GnRH or estradiol to facilitate fixed-time insemination in an MGA-based synchronization regimen in beef cattle M.F. Martinez a , J.P. Kastelic b , G.P. Adams a , R.J. Mapletoft a,* a Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, 52 Campus Drive, Saskatoon, Sask., Canada S7N 5B4 b Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, Alta., Canada T1J 4B1 Received 15 August 2000; received in revised form 11 June 2001; accepted 14 June 2001 Abstract Two experiments were conducted to compare pregnancy rates when GnRH or estradiol were given to synchronize ovarian follicular wave emergence and ovulation in an MGA-based estrus synchro- nization program. Crossbred beef cattle were fed melengestrol acetate (MGA, 0.5 mg per day) for 7 days (designated days 0–6, without regard to stage of the estrous cycle) and given cloprostenol (PGF; 500 g intramuscular (im)) on day 7. In Experiment 1, lactating beef cows (n = 140) and pubertal heifers (n = 40) were randomly allocated to three groups to receive 100 g gonadorelin (GnRH), 5 mg estradiol-17 and 100 mg progesterone (E + P) in canola oil or no treatment (control) on day 0. All cattle were observed for estrus every 12 h from 36 to 96 h after PGF. Cattle in the GnRH group that were detected in estrus 36 or 48 h after PGF were inseminated 12 h later; the remainder were given 100 g GnRH im 72 h after PGF and concurrently inseminated. Cattle in the E + P group were randomly assigned to receive either 0.5 or 1.0 mg estradiol benzoate (EB) in 2 ml canola oil im 24 h after PGF and were inseminated 30 h later. Cattle in the control group were inseminated 12 h after the first detection of estrus; if not in estrus by 72 h after PGF, they were given 100 g GnRH im and concurrently inseminated. In the absence of significant differences, all data for heifers and for cows were combined and the 0.5 and 1.0 mg EB groups were combined into a single estradiol group. Estrus rates were 57.6, 57.4 and 60.0% for the GnRH, E + P and control groups, respectively (P = 0.95). The mean (±S.D.) interval from PGF treatment to estrus was shorter (P< 0.001) and less variable (P< 0.001) in the E + P group (49.0 ± 6.1 h) than in either the GnRH (64.2 ± 15.9 h) or control (66.3 ± 13.3 h) groups. Overall pregnancy rates were higher (P< 0.005) in the GnRH (57.6%) and E + P (55.7%) groups than in the control group (30.0%) as were pregnancy rates to fixed-time AI (47.5, 55.7 and 28.3%, respectively). In Experiment 2, 122 * Corresponding author. Tel.: +1-306-966-7149; fax: +1-306-966-7159. E-mail address: reuben.mapletoft@usask.ca (R.J. Mapletoft). 0378-4320/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0378-4320(01)00128-2