76 Effect of length of proestrus on pregnancies per embryo transfer and pregnancy losses in beef recipients synchronised with estradiol/progesterone-based protocols

Although the success of commercial embryo transfer is dependent on the number of live offspring obtained, most studies only determine pregnancy rates and do not report how many pregnancies are carried to term. An experiment was designed to evaluate the effect of length of proestrus on pregnancies per embryo transfer (P/ET), pregnancy losses, and calving rates in beef recipients synchronised with oestradiol/progesterone (P4)-based protocols. Multiparous, non-lactating Bos indicus crossbred beef cows (n = 750), with a corpus luteum (CL) or at least one follicle >8 mm in diameter and a body condition score between 2.5 and 4 (1 to 5 scale) were synchronised with two prolonged proestrus (J-Synch) or one short proestrus (ECP) protocol. On Day 0, all cows received 2 mg of oestradiol benzoate (EB; Gonadiol, Zoetis) and an intravaginal device (0.5 g of P4; DIB, Zoetis). The P4 device was removed on Day 6 in the J-Synch 6 group and on Day 7 in the J-Synch 7 and ECP groups. All cows received 500 µg of cloprostenol (Ciclase DL, Zoetis) and 400 IU of equine chorionic gonadotrophin (eCG; Novormon 5000, Zoetis) at the time of P4 device removal, and cows in the ECP group also received 1 mg of oestradiol cypionate (Cipiosyn, Zoetis) at that time. All cows had their tails painted for oestrus detection (CeloTest, Biotay). Cows with ≥50% tail paint removed by 72 h (J-Synch groups) or 48 h (ECP group) after P4 device removal were considered in oestrus. All cows in oestrus were examined by ultrasonography 7 days later and those with a CL >16 mm in diameter received a fresh in vitro-produced embryo by nonsurgical transfer. Pregnancy was determined at 30 and 90 days by ultrasonography, and all pregnant recipients were monitored until parturition to determine pregnancy losses during gestation. Data were analysed using the generalised mixed model procedure for binary data and a logit link (InfoStat, 2021). The proportion of synchronised cows that were transferred was greater (P < 0.05) in the ECP group (89.0%, 222/250) than in the J-Synch 6 (81.0%, 203/250) or J-Synch 7 (82.0%, 205/250) groups. However, 30-day P/ET did not differ (P > 0.2) among the ECP (41.8% 92/222), J-Synch 6 (42.8%, 87/203) and J-Synch 7 (40.0%, 82/205) groups. Although 90-day P/ET did not differ among groups (ECP: 35.1%, 78/222; J-Synch 6: 38.9%, 79/203; and J-Synch 7: 37.0%, 76/205), 30- to 90-day pregnancy losses were greater (P < 0.04) in the ECP group (16.1%, 15/93) than in the J-Synch 6 (7.0%, 6/82) or J-Synch 7 (9.0%, 8/87) groups. Pregnancy losses from 90 days to parturition were also greater (P < 0.05) in the ECP group (15.3%, 12/78) than in the J-Synch 6 (5.0% 4/79) and J-Synch 7 (3.0% 3/76) groups. Consequently, calving rate was lower (P < 0.05) in the ECP group (29.7%, 66/222) than in the J-Synch 6 (36.9%, 75/203) and J-Synch 7 (35.6%, 73/205) groups. In summary, synchronisation protocols with longer proestrus did not improve P/ET but were associated with lesser pregnancy losses and greater calving rates in recipients treated with oestradiol/P4-based synchronisation protocols.