110 Pregnancy rates in Bos indicus × Bos taurus recipients synchronized with a GnRH/progesterone-based or an estradiol/progesterone-based protocol with prolonged proestrus

An experiment was designed to compare utilisation and pregnancy rates to embryo transfer (P/ET) in Bos indicus × Bos taurus heifers (Gyr × Jersey) synchronized with two protocols with prolonged proestrus, an oestradiol/progesterone (P4)-based (J-Synch) or a GnRH/progesterone-based (Web-Synch). Pubertal (2-year-old) heifers with a CL (determined by ultrasonography), body condition score between 3.5 and 4 (1–5 scale) and managed in a grazing system, were randomly allocated into one of two treatment groups. On Day 0, heifers in the J-Synch group (n = 375) received 2 mg of oestradiol benzoate (EB, Calier, Argentina) and an intravaginal device containing 0.5 g of P4 (DIB 0.5, Zoetis, Argentina). On Day 6, P4 devices were removed, and all heifers received 150 µg of D-cloprostenol (PGF, Veteglan, Calier) and 300 IUof equine chorionic gonadotrophin (eCG; Vetegon, Calier). Heifers in the Web-Synch group (n = 381) were treated with PGF and a P4 device on Day −5, 10 µg of buserelin (gonadotrophin-releasing hormone, GnRH; Pluserelina, Calier) on Day 0 and P4 device removal, PGF and eCG on Day 6. All heifers were observed for signs oestrus, which was based on tail-paint removal (>50%), by 72 h after P4 device removal. In those detected in oestrus their ovaries were examined by ultrasonography (Mindray M6 Vet) on Day 16 and those with a CL >16 mm in diameter received a grade 1 in vitro-produced blastocysts by nonsurgical transfer. Recipients without their tail-paint removed were not transferred. Pregnancy was determined by ultrasonography 30 and 90 days after embryo transfer. Data were analysed using the GLM mixed procedure for binary data with a logit link (InfoStat, 2022) and results are presented in Table 1. The utilisation and P/ET were greater, and the pregnancy losses were lesser in heifers in the J-Synch group compared to those in the Web-Synch group. It was concluded that the oestradiol/progesterone-based protocol is more appropriate than the GnRH/based protocol evaluated in this study for the synchronization of Bos indicus × Bos taurus heifers used as embryo recipients.