Effects of progesterone presynchronization and eCG on pregnancy rates to GnRH-based, timed-AI in beef cattle § J.A. Small a, * , M.G. Colazo b , J.P. Kastelic c , R.J. Mapletoft d a Agriculture and Agri-Food Canada, Brandon, Manitoba, Canada R7A 5Y3 b Alberta Agriculture and Food, Edmonton, Alberta, Canada T6H 5T6 c Agriculture and Agri-Food Canada, Lethbridge, Alberta, Canada T1J 4B1 d Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B4 Received 9 February 2008; received in revised form 12 June 2008; accepted 12 September 2008 Abstract Three experiments were conducted to determine the effects of low-dose progesterone presynchronization and eCG on pregnancy rates to GnRH-based, timed-AI (TAI) in beef cattle (GnRH on Day 0, PGF 2a on Day 7, with GnRH and TAI on Day 9, 54–56 h after PGF 2a ). Experiments 1 and 2 were 2  2 factorials with presynchronization (with or without a once-used CIDR; Days 15 to 0 in Experiment 1 and Days 7 to 0, with PGF 2a at insertion, in Experiment 2), and with or without 400 IU eCG on Day 7 in suckled cows. In Experiment 3, suckled cows and nulliparous heifers were either presynchronized with a twice-used CIDR (Days 5 to 0) and PGF 2a at insertion, or no treatment prior to insertion of a new CIDR (Days 0–7). Presynchronization increased (P < 0.05) ovulation rate to GnRH on Day 0 (75.0% vs 48.7%, 76.7% vs 55.0%, and 60.0% vs 36.1% for Experiments 1, 2, and 3, respectively), increased the diameter of the preovulatory follicle in Experiments 1 and 2, and increased the response to PGF 2a (regardless of parity) in Experiment 1 (P < 0.01), and in primiparous cows in Experiment 2 (P < 0.01). Effects of presynchronization on pregnancy rates (53.4% vs 54.1%, 57.7% vs 45.3%, and 54.3% vs 44.4% for Experiments 1, 2, and 3, respectively) were influenced by parity and eCG (P < 0.05). Treatment with eCG had no effect (P > 0.05) on the diameter of the preovulatory follicle (Experiment 1), or the response to PGF 2a (Experiments 1 and 2), but tended (P = 0.08) to improve pregnancy rates, especially in primiparous cows that were not presynchronized (P < 0.01). However, the effects of eCG and presynchronization were not additive. # 2009 Published by Elsevier Inc. Keywords: Estrus synchronization; Timed-AI; Presynchronization; Ovarian follicular development; Cattle 1. Introduction The basic GnRH-based protocol for timed-AI (TAI) of cattle consists of an initial treatment with GnRH, PGF 2a 7 d later, and a second GnRH treatment 48 h later, either 16–18 h before (Ovsynch) or concurrent with (CO-Synch) TAI [1,2]. Pregnancy rates were low when ovulation did not occur in response to the first GnRH and estrus was premature, [3,4], or the second GnRH induced ovulation of small (<12 mm) pre- ovulatory follicles [5]. Conversely, pregnancy rates in www.theriojournal.com Available online at www.sciencedirect.com Theriogenology 71 (2009) 698–706 § Portions of these data were presented at the Annual Meetings of the International Embryo Transfer Society, Copenhagen, Denmark January 2005 and Orlando, FL, USA, January 2006. * Corresponding author. Current address: Haley Institute, Depart- ment of Plant and Animal Sciences, Nova Scotia Agricultural College, Truro, Nova Scotia, Canada B2N 5B3. Tel.: +1 902 896 2499; fax: +1 902 895 6734. E-mail addresses: jsmall@agr.gc.ca, jsmall@nsac.ca (J.A. Small). 0093-691X/$ – see front matter # 2009 Published by Elsevier Inc. doi:10.1016/j.theriogenology.2008.09.045