147 USE OF EQUINE CHORIONIC GONADOTROPIN AFTER EMBRYO TRANSFER IN NELORE AND CROSSBRED RECIPIENT CATTLE

Abstract

Recent studies that have used eCG administration either 14 or 22 days after fixed-time AI in dairy and beef cattle have indicated a positive effect on conception rates. The aim of this study was to evaluate conception rates in embryo recipients treated with eCG 7 days after fixed-time embryo transfer (ET). A protocol for synchronization of ovulation was used in 470 Nelore and crossbred embryo recipient heifers (n = 179), dry cows (n = 152), and lactating cows (n = 139; body condition score = 3.0 to 3.5; >40 days in milk). All heifers and dry cows, but only 27.3% of lactating cows, were cycling at the beginning of the synchronization protocol. On Day 0, each female received IM injections of 2 mg of oestradiol benzoate (Gonadiol, Intervet Schering-Plough Animal Health, São Paulo, Brazil) and 0.075 mg of PGF_2α (Preloban, Intervet Schering-Plough Animal Health). An intravaginal progesterone-releasing device (DIB, Intervet Schering-Plough Animal Health) was also inserted on Day 0. On Day 8, the DIB device was removed and cows or heifers received another IM treatment of 0.075 mg of PGF_2α (Preloban), 300 IU of eCG (Novormon, Syntex S.A., Buenos Aires, Argentina) and 0.6 mg of oestradiol cipionate (Pfizer, Brazil). Transrectal ultrasonography was performed in all recipients on the day of ET (Day 16, 17, or 18 of the protocol) and only those with a corpus luteum greater than 15 mm in diameter were selected (n = 113 heifers, 114 dry cows, and 70 lactating cows). Transfers of fresh, frozen, and vitrified in vivo-produced embryos from Nelore donors were balanced between experimental groups. Seven days after ET, 127 animals received an IM injection of 400 IU of eCG (eCG group), and the 170 remaining cows or heifers were not treated with eCG, and were therefore considered the control group. Pregnancy diagnosis was performed 30 and 60 days after ovulation by transrectal ultrasound. Data were analysed using generalized linear models and results are presented as least squares means ± standard error. Conception rates were similar in the eCG and control groups at Day 30 (35.9 ± 5.5 and 33.5 ± 4.4, respectively; P > 0.10) and at Day 60 (27.6 ± 4.9 and 26.7 ± 3.9, respectively; P > 0.10). Contrary to other results in the literature, especially in dairy cows bred by fixed-time AI, the use of eCG 7 days after ET did not improve conception rates in Bos indicus cattle used as embryo recipients.