Resynchronization with unknown pregnancy status using progestin- based timed artificial insemination protocol in beef cattle M.F. Sá Filho a, * , M.O. Marques b , R. Girotto c , F.A. Santos c , R.V. Sala a , J.P. Barbuio d , P.S. Baruselli a a Departamento de Reprodução Animal, FMVZ/USP, São Paulo, SP, Brazil b Geraembryo, Cornélio Procópio, PR, Brazil c RG Genética Avançada, Agua Boa, MT, Brazil d MSD Animal Health, São Paulo, Brazil article info Article history: Received 27 June 2013 Received in revised form 21 September 2013 Accepted 23 September 2013 Keywords: Cattle Bos indicus Estrous synchronization Timed artificial insemination abstract Two experiments were designed to evaluate the use of resynchronization (RESYNCH) protocols using a progestin-based timed artificial insemination (TAI) protocol in beef cattle. In experiment 1, 475 cyclic Nelore heifers were resynchronized 22 days after the first TAI using two different inducers of new follicular wave emergence (estradiol benzoate [EB; n ¼ 241] or GnRH [n ¼ 234]) with the insertion of a norgestomet ear implant. At ear implant removal (7 days later), a pregnancy test was performed, and nonpregnant heifers received a dose of prostaglandin plus 0.5 mg of estradiol cypionate, with a timed insemination 48 hours later. The pregnancy rate after the first TAI was similar (P ¼ 0.97) between treat- ments (EB [41.9%] vs. GnRH [41.5%]). However, EB-treated heifers (49.3%) had a greater (P ¼ 0.04) pregnancy per AI (P/AI) after the resynchronization than the GnRH-treated heifers (37.2%). In experiment 2, the pregnancy loss in 664 zebu females (344 nonlactating cows and 320 cyclic heifers) between 30 and 60 days after resynchronization was evaluated. Females were randomly assigned to one of two groups (RESYNCH 22 days after the first TAI [n ¼ 317] or submitted only to natural mating [NM; n ¼ 347]). Females from the NM group were maintained with bulls from 15 to 30 days after the first TAI. The RESYNC-treated females were resynchronized 22 days after the first TAI using 1 mg of EB on the first day of the resynchronization, similar to experiment 1. No difference was found in P/AI (NM [57.1%] vs. RESYNC [61.5%]; P ¼ 0.32) or pregnancy loss (NM [2.0%] vs. RESYNC [4.1%]; P ¼ 0.21) after the first TAI. Moreover, the overall P/AI after the RESYNCH protocol was 47.5%. Thus, the administration of 1 mg of EB on day 22 after the first TAI, when the pregnancy status was undetermined, promotes a higher P/AI in the resynchronized TAI than the use of GnRH. Also, the administration of 1 mg of EB 22 days after the TAI did not affect the preestablished pregnancy. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction The incorporation of biotechnologies, such as artificial insemination (AI), to improve the genetic gains in calf operation systems is essential. To accomplish these genetic gains, the reproductive programs must focus on enhancing the proportion of cows calving from AI by improving the service rates and reducing the interval between in- seminations without compromising the survival of the embryo/fetus. Tropical countries have a significant participation in the world’s beef production. Zebu cattle are the main breed raised in these areas. Furthermore, there are some repro- ductive physiologic differences between Bos taurus and * Corresponding author. Tel./Fax: þ55 (11) 3091 7674. E-mail address: manoelsa@usp.br (M.F. Sá Filho). Contents lists available at ScienceDirect Theriogenology journal homepage: www.theriojournal.com 0093-691X/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.theriogenology.2013.09.027 Theriogenology 81 (2014) 284–290