166 COMPARISON OF TWO METHODS TO SYNCHRONIZE CROSSBRED BEEF HEIFERS FOR EMBRYO TRANSFER

Abstract

Estrous synchronization with prostaglandin in commercial MOET programs frequently results in insufficient numbers of recipients for embryo transfer. The aim of this trial was to compare the efficiency of 2 different methods of estrous synchronization in crossbred beef heifers. A total of 235 cycling heifers (Brangus and Braford), with a mean age of 24 months (range 22-26 months), body condition score of 3.5 ± 0.1 (1-5 scale), and mean weight of 340 ± 22 kg were assigned to 2 treatment groups stratified according to body weight and genotype. In Group 1 (n = 157), heifers received 2 injections of prostaglandin [Croniben, 150 μg of D(+) cloprostenol, i.m.; Biogenesis-Bágo, Buenos Aires, Argentina] with an interval of 14 days. After the second injection, detection of estrus was monitored for 5 days. In Group 2 (n = 78), heifers received an intravaginal device of progesterone (Day 0; 0.558 g; Cronipres Mono Dose M-24, Biogenesis-Bágo) and at the same time received an i.m. injection of estradiol benzoate (2 mg; Cronibest, Biogenesis-Bágo). On Day 7, devices were removed and heifers were injected with prostaglandin. Twenty-four hours later (Day 8), 1 mg of estradiol benzoate was injected i.m. All recipients had estrus detected for 72 h after the end of treatment to determine the estrous synchronization rate. Before embryo transfer 7 days after the detected estrus, heifers were examined by transrectal ultrasonography (Chison 600Vet, 5-MHz transducer, National Ultrasound, Duluth, GA, USA) to confirm the presence of a CL and its size or an unovulated follicle on the ovaries. Those with a CL of 1.7 cm² or greater were selected to receive a frozen-thawed embryo. The frequencies of synchronous estrus, ovulation, unovulated follicles, and use of recipients were compared by chi-square test. Group 2 had a higher estrous synchronization rate (70%) and proportion of selected recipients (51.2%) compared with Group 1 (55% and 35.6%, respectively; P < 0.05). Ovulation rate (64% v. 72%) and incidence of unovulated follicles (20.6% v. 21.8%) for Groups 1 and 2, respectively, did not differ (P > 0.05). In summary, treatment with a 0.558-g progesterone intravaginal device, plus injections of estradiol benzoate and PGF, was more efficient than 2 PGF injections alone in synchronizing estrus and increased the use of crossbred beef heifer recipients in MOET programs. The ovulation rate and incidence of unovulated follicles were similar in both methods of estrous synchronization.