Effect of length of progesterone exposure during ovulatory wave development on pregnancy rate F.C.F. Dias a , R.J. Mapletoft b , J.P. Kastelic c , G.P. Adams a , M.G. Colazo d , B.C. Stover a , O. Dochi e , J. Singh a, * a Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada b Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada c Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, AB, Canada d Alberta Agriculture and . Rural Development, Edmonton, Alberta, Canada e Department of Dairy Science, Rakuno Gakuen University, Ebetsu, Hokkaido, Japan Received 4 February 2011; received in revised form 11 August 2011; accepted 16 August 2011 Abstract The objective was to determine the effects of the duration of progesterone exposure during the ovulatory wave on fertility (pregnancy rate) in beef cattle. We tested the hypothesis that short-progesterone exposure during the growing and early-static phase of the ovulatory follicle (analogous to the ovulatory wave of 3-wave cycles) is associated with higher fertility than a longer duration of exposure (analogous to the ovulatory wave of 2-wave cycles). Three to 5 days after ovulation, beef heifers (n = 172) and suckled beef cows (n = 193) were given an intravaginal progesterone-releasing device (CIDR) and 2.5 mg estradiol - 17 +50 mg progesterone im to induce a new follicular wave. Cattle were allocated to short- or long-progesterone exposure groups (for 3 and 6 d after wave emergence, respectively) after which prostaglandin F 2 was administered and CIDR were removed. Forty-eight hours later, all cattle were given 12.5 mg pLH and artificially inseminated (AI) with frozen-thawed semen. The diameter of the two largest follicles and the corpus luteum were measured by transrectal ultrasonography at CIDR removal, insemination, and 36 h after insemination. Pregnancy diagnosis was done ultrasonically 38 and 65 d post-AI. There was no difference in pregnancy rates in short- vs long-progesterone exposure in heifers (53 vs 47%, P = 0.44) or cows (63 vs 58%, P = 0.51). However, the diameter of the ovulatory follicle at CIDR removal and AI was smaller in short- than in long-progesterone groups (P  0.02), and larger in cows than in heifers (P  0.006). In conclusion, short-progesterone exposure during the growing and early-static phase of the ovulatory follicle (similar to 3-wave cycles) was not associated with higher fertility than a longer progesterone exposure (similar to 2-wave cycles). Crown Copyright © 2012 Published by Elsevier Inc. All rights reserved. Keywords: Follicular waves; Heifer; Ovarian follicle; Progesterone; Postpartum cows 1. Introduction Follicles develop in a wave-like pattern, with most cattle having either 2 or 3 follicular waves during each estrous cycle [1]. The duration of the ovulatory wave in 2-wave cycles is approximately 9 d, whereas the ovu- * Corresponding author. Tel.: 1-306 966 7410; fax: 1-606 966 7405. E-mail address: jaswant.singh@usask.ca (J. Singh). Available online at www.sciencedirect.com Theriogenology 77 (2012) 437– 444 www.theriojournal.com 0093-691X/$ – see front matter Crown Copyright © 2012 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.theriogenology.2011.08.024