Comparison of controlled internal drug release device and melengesterol acetate as progestin sources in an estrous synchronization protocol for beef heifers S.A. Tauck, J.R.C. Wilkinson, J.R. Olsen, J.N. Janitell, J.G. Berardinelli * Department of Animal and Range Sciences, Montana State University, Bozeman 59717, United States Received 28 December 2006; accepted 24 March 2007 Abstract The objectives of this experiment were to compare estrous synchronization responses and AI pregnancy rates of beef heifers using protocols that included either CIDR or MGA as the progestin source. The hypotheses tested were that: (1) estrous synchronization responses after (a) progestin removal, and (b) PGF 2a ; and, (2) AI pregnancy rates, do not differ between heifers synchronized with either progestin source. At the start of the experiment (Day 0) in both years, heifers were assigned randomly to receive, MGA supplement for 14 days (MGA-treated; n = 79) or CIDR for 14 days (CIDR-treated; n = 77). On Day 14 progestin was removed and heifers were observed for estrus up to and after PGF 2a on Days 31 and 33 for CIDR-treated and MGA-treated heifers, respectively. Heifers that exhibited estrus within 60 h after PGF 2a were inseminated by AI 12 h later; the remaining heifers were inseminated at 72 h after PGF 2a and given GnRH (100 mg). More (P < 0.05) CIDR-treated heifers exhibited estrus within 120 h after progestin removal than MGA-treated heifers. Intervals to estrus after progestin removal were shorter (P < 0.05) for CIDR-treated heifers than MGA-treated heifers. More (P < 0.05) CIDR-treated heifers exhibited estrus and were inseminated within 60 h after PGF 2a than MGA-treated heifers. Pregnancy rates did not differ (P > 0.10) between MGA-treated (66%) and CIDR-treated (62%) heifers. In conclusion, the use of CIDR as a progestin source in a 14-day progestin, PGF 2a , and timed AI and GnRH estrous synchronization protocol was as effective as the use of MGA to synchronize estrus and generate AI pregnancies in beef heifers. # 2007 Elsevier Inc. All rights reserved. Keywords: Controlled internal drug release; Estrous synchronization; Fixed-time AI; Melengesterol acetate; Beef heifers 1. Introduction Lifetime productivity increases if heifers are insemi- nated and conceive early in their first breeding season [1]. Progestin treatment for 9 or 14 days of melenges- terol acetate (MGA) can induce puberty in yearling beef heifers [2,3]. Estrous synchronization protocols that include progestin treatment for at least 9 days can be an effective method to increase the proportion of heifers that become pregnant early in their first breeding season. There are many methods for estrous synchro- nization available to producers. The most widely used and accepted estrous synchronization method for yearling heifers is feeding MGA for 14 days, followed by PGF 2a 19 or 17 days later [4,5]. For this estrous synchronization protocol to be successful, each heifer must consume a daily dosage of 0.5 mg/day of MGA [6]. Unfortunately, many cow-calf producers lack the facilities, time, and labor necessary to successfully www.theriojournal.com Theriogenology 68 (2007) 162–167 * Corresponding author. Tel.: +1 406 994 5574; fax: +1 406 994 5589. E-mail address: jgb@montana.edu (J.G. Berardinelli). 0093-691X/$ – see front matter # 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.theriogenology.2007.03.027