Effects of progesterone presynchronization and eCG on pregnancy rates to GnRH-based, timed-AI in beef cattle》.pdf

Available online at Theriogenology 71 (2009) 698–706 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, PGF2a on Day 7, with GnRH and TAI on Day 9, 54–56 h after PGF2a). 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 PGF2a 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 PGF2a 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 P