122 Pregnancy rates, pregnancy losses, and calving rates in Bos indicus heifers used as embryo recipients and synchronized with a GnRH-based or an estradiol-based protocol

An experiment was designed to compare pregnancy rates to embryo transfer (P/ET), pregnancy losses, and calving rates in Bos indicus heifers (Brahman) synchronized with two protocols with prolonged proestrus, one estradiol/progesterone (P4)-based (J-Synch) and the other GnRH/P4-based (Web-Synch). Brahman heifers, aged 2.5 years (n = 230), were randomly allocated into one of two treatment groups. In both groups, heifers had a CL (determined by ultrasonography) and a body condition score of 3–4 (1–5 scale) and were managed in a grazing system. On Day 0, heifers in the J-Synch group (n = 115) received 2 mg estradiol benzoate (Calier) and an intravaginal device containing 0.5 g P4 (DIB 0.5, Zoetis). On Day 6, P4 devices were removed, and all heifers received 150 µg of d-cloprostenol (PGF, Ciclase DL, Zoetis) and 300 IU eCG (Novormon, Zoetis). Heifers in the Web-Synch group (n = 115) were treated with PGF and a P4 device on Day −5; 10 µg of buserelin (GnRH, Pluserelina, Calier) on Day 0; and P4 device removal, PGF, and eCG on Day 6. All heifers were observed for estrus signs 72 h after device removal, and those detected in estrus were examined by ultrasonography (Mindray M6 Vet) on Day 16. Recipients with a CL > 16 mm in diameter received grade 1 in vitro–produced blastocysts by nonsurgical ET. Blood samples were collected from selected heifers at the time of ET to determine plasma progesterone concentrations. Pregnancy was determined by ultrasonography at 30 and 90 days of gestation, and heifers were followed until parturition to determine calving rates. Data were analyzed using the GLM mixed procedure for binary data with a logit link (InfoStat, Di Rienzo et al. 2022), and results are presented in Table 1. Although estrus and utilization rates did not differ among groups, heifers in the J-Synch group had greater (P < 0.05) plasma P4 concentrations, 30- and 90-day P/ET and calving rates, and fewer (P < 0.05) 30- to 90-day pregnancy losses than those in the Web-Synch group. It was concluded that the estradiol/P4-based protocol is more appropriate than the GnRH/P4-based protocol evaluated in this study for the synchronization of Bos indicus heifers used as embryo recipients.