66 † Corresponding author © 2016 Conscientia Beam. All Rights Reserved. THE EFFECT OF THE REDUCED DOSE OF GnRH ON CONCEPTION, OVULATION AND OVARIAN STRUCTURES IN OVSYNCH PROGRAM OF LACTATING DAIRY COWS Fikret Karaca 1† --- Gokhan Dogruer 2 --- Mustafa Kemal Saribay 3 --- Yasar Ergun 4 --- Cafer Tayyar Ates 5 1 Mustafa Kemal University, Faculty of Veterinary Medicine, Department of Reproduction and Artificial Insemination, Hatay 2,3,4 Mustafa Kemal University, Faculty of Veterinary Medicine, Department of Obstetrics and Gynecology, Hatay 5 Mustafa Kemal University, Faculty of Veterinary Medicine, Department of Animal Breeding, Hatay ABSTRACT This study was conducted to evaluate the efficacy of reducing GnRH dose on the formation of ovulation and conception, and sizes of the ovarian structures following an Ovsynch program in lactating cows. The cows were allocated randomly to two treatment groups (full dose; FD, n=20 and half dose; HD, n=20). Cows in the FD group were treated with 10.5 µg buserelin acetate on day 0, with 0.150 mg D- cloprostenol 7 d later and with 10.5 µg buserelin acetate 2 d later. Estrous cycles in HD group were synchronized using the same scheme as FD-treated cows, but the dose of buserelin acetate was reduced to 5.25 µg at both GnRH administration times. Ovarian structures were monitored by ultrasound with a 6-8 MHz linear trans-rectal probe on days 0, 7, 9, 10, and 11. Cows were inseminated at the 16-20 h after second GnRH administration. No significant differences were observed in the dominant or ovulatory follicle diameters in FD and HD groups. Ovulation incidence from second GnRH injection by the 24 hour after fixed-time AI did not differ between FD (85 %) and HD (90 %) groups. Also, the conception percentages did not differ statistically between the HD (50%) and FD (40 %) groups. Keywords: Buserelin acetate, Ovsynch, Ovarian structures, Ovulation, Conception, Dairy cows. Received: 16 November 2016/ Revised: 17 December 2016/ Accepted: 30 December 2016/ Published: 7 January 2017 Contribution/ Originality This study is one of very few studies which have investigated the effect of reducing doses of buserelin acetate on the follicular development, ovulation and conception rates in the Ovsynch program of dairy cows. 1. INTRODUCTION The goal for a successful estrous synchronization program in lactating dairy cattle is the precise control of estrous, which will allow high fertility to a fixed-timed artificial insemination (FTAI) without the need for estrous detection [1]. Previous studies, performed with PGF 2α or progesterone based estrous synchronization programs in dairy cows, have shown that these programs are not adequate for FTAI without estrous detection [2, 3]. In 1995, a hormonal protocol was developed to synchronize ovulation (Ovsynch) in lactating dairy cows using GnRH and PGF 2α . Ovsynch protocol may have a major impact on managing reproduction of lactating dairy cows, since it can permit FTAI without the need for estrous detection [4]. In the Ovsynch program, GnRH is given at a random stage of the estrous cycle, followed by PGF 2α on day 7 and second a dose of GnRH 48 h later [4, 5]. This program coordinates follicular recruitment, corpus luteum (CL) regression and time of ovulation also permits timed Animal Review 2016 Vol. 3, No. 3, pp. 66-72 ISSN(e): 2409-6490 ISSN(p): 2412-3382 DOI: 10.18488/journal.ar/2016.3.3/101.3.66.72 © 2016 Conscientia Beam. All Rights Reserved.