The ovulatory response to human chorionic gonadotropin administration on day 4 post timed artificial insemination improved fertility in repeat breeder cows

This study focused on determining the success of inducing endogenous progesterone source (by formation of accessory corpus luteum) by human chorionic gonadotropin (hCG) administration on day 4 post timed artificial insemination (TAI) and on evaluating the effect of the day of hCG administration (day 4 or 5 post TAI) on pregnancy rates in repeat breeder cows according to their responses to hCG administrations. Non-pregnant cyclic lactating Holstein dairy cows with more than three services despite no clinical reproductive abnormalities were enrolled in the study. All cows (n = 192) received the Ovsynch protocol (GnRH-7d-PGF2α-56h-GnRH-16/18h-TAI) and hCG administration (1500 IU, i.m.) was performed in group hCG4 (n = 64) on day 4 and in group hCG5 (n = 68) on day 5 post TAI. The control group (CON, n = 60) did not receive any hCG treatment. The responses (ovulation rate) to hCG (~64%) were similar on days 4 and 5 post TAI. Pregnancy per AI (P/AI) at 31 days was similar among the groups (48.3% in CON, 48.4% in hCG4, and 42.6% in hCG5). Regardless of the day of the treatment (hCG4 + hCG5), the cows had an ovulatory response to hCG (54.1%), and a greater P/AI (P = 0.007) at 31 days than those without a response (29.8%). The administration of hCG on either day 4 or 5 post TAI did not increase overall pregnancy rates, however, the ovulatory response to the post TAI hCG administration on day 4 improved fertility in repeat breeder cows. Further studies are needed with a larger number of animals to reveal this potentiallly beneficial effect. Cattle, repeat breeding, hCG, post TAI Repeat breeder (RB) cows are generally defined as cows that have no pregnancy after more than three services despite normal cyclicity and healthy genital tracts (Zemjanis 1980). Repeat breeding causes important financial loss on dairy farms because of increased calving intervals, culling rates, and insemination costs. Since there are multiple factors that correlate with repeat breeding in cows, the specific reasons remain speculative (Gustafsson and Emanuelson 2002). Most prominent aetiological factors include oestrus detection errors, infections, and hormonal disturbances leading to embryonic mortalities (Gustafsson and Emanuelson 2002; Opsomer 2014). Hormonal disturbances such as delayed progesterone increases (Shelton et al. 1990; Bage et al. 2002; Moss et al. 2002; Kendall et al. 2009) and decreased total concentrations of progesterone after ovulation (Shelton et al. 1990; Bage et al. 2002) may induce embryonic mortalities which occur mostly 6 to 8 days after timed artificial insemination (TAI) in RB cows (Shelton et al. 1990). Since progesterone plays a critical role in embryonic development and continuation of pregnancy, several studies have focused on either exogenous or endogenous progesterone supplementation during the post-TAI period to increase the pregnancy rate (Tefera et al. 2001; Mann et al. 2006; Khoramian et al. 2011; Wiltbank et al. 2011). Endogenous progesterone can be stimulated by induction of ovulation of the dominant follicle generated ACTA VET. BRNO 2021, 90: 015–020; https://doi.org/10.2754/avb202190010015 Address for correspondence: Gulnaz Yilmazbas-Mecitoglu Department of Obstetrics and Gynecology Faculty of Veterinary Medicine, University of Bursa Uludag Gorukle Campus, 16059, Bursa, Turkey Phone: +90 224 294 08 27 E-mail: gulnazy@uludag.edu. tr http://actavet.vfu. cz/