170 Embryonic loss and pregnancy rate in response to resynchronisation using oestradiol benzoate or injectable progesterone at 14 days after timed AI in Bos taurus × Bos indicus beef heifers

Abstract

The effect of the exposure of pregnant cattle to oestradiol benzoate (EB) or injectable long-acting progesterone (iP4), as well as their use for early resynchronisation of non-pregnant (NP) females within the second week after timed AI (TAI), is yet to be assessed. This study aimed to assess the pregnancy rate and embryonic loss of cross-bred Bos taurus × Bos indicus beef heifers in response to an EB- or iP4-based resynchronisation strategy implemented 14 days after the first TAI. A total of 677 cyclic heifers (Braford and Brangus; 24 months old) were subjected to an EB/P4 synchronisation protocol for the first TAI (Day 0 = TAI). On Day 14, all heifers received a new P4 intravaginal device (1 g, Sincrogest, Ourofino Saúde Animal) and were randomly assigned to two groups: EB (1 mg intramuscularly, Sincrodiol; n = 339) or iP4 (75 mg intramuscularly, Sincrogest Injetável; n = 338). On Day 22, P4 devices were removed and heifers underwent colour Doppler ultrasonography evaluation. Non-pregnant females were identified by the assessment of structural regression of the corpus luteum, as previously reported by our group. On Day 22, NP heifers received 1 mg of oestradiol cipionate (SincroCP), 250 µg of sodium cloprostenol (Sincrocio), and 200 IU of equine chorionic gonadotrophin (SincroeCG), and they were inseminated on Day 24. On Day 22, the diameter of the preovulatory follicle (POF) was also recorded. Pregnancy was diagnosed via B-mode ultrasonography on Days 35-40 and 54. Statistical analyses were performed using SAS software (ver. 9.2, SAS Institute Inc.). The MIXED procedure was used to evaluate the POF diameter, and the GLIMMIX procedure was used to develop a multivariate model including treatment, breed, age, farm (1-7), and possible interactions of treatment with farm, breed, and age to evaluate the binomial data (pregnancy rates and embryonic loss). In NP heifers, the diameter of the POF on Day 22 did not differ (P > 0.05) between the EB (9.77 ± 0.25 mm) and iP4 (9.92 ± 0.22 mm) groups. Exposure to EB or iP4 on Day 14 post-TAI did not influence pregnancy rates as assessed on Day 22 (56.3% (191/339) vs. 60.1% (203/338)) and on Days 35-40 (49.6% (168/339) vs. 53.3% (180/338)). In addition, embryonic loss between Days 22 and 35-40 was not different between the EB (12%, 23/191) and iP4 (11.3%, 23/203) groups. On the other hand, pregnancy rates in heifers submitted to the second TAI were greater (P = 0.01) in the EB group (45.95%, 68/148) than in the iP4 group (31.85%, 43/135). In conclusion, improved pregnancy rates can be achieved using 1 mg of EB associated with a P4 intravaginal device on Day 14 of the resynchronisation protocol without affecting the first TAI pregnancy rates and embryonic loss. However, further investigation is warranted to assess the lower pregnancy rates obtained in response to the association of iP4 and P4 device.

This research was supported by FAPESP (2015/10606-9; 2019/15514-6), Graduate Studies Support Program (PAPG-UNIPAMPA), and Ourofino Saúde Animal.