PSX-4 Reproductive outcomes in Nelore beef heifers submitted to different protocols using injectable progesterone to induce ovulation prior to the timed-AI protocol.

Abstract Reduction of age at first calving is a factor that contributes to the improvement of reproductive efficiency in cow-calf systems. As in Nelore females the puberty attainment occurs late, strategies such as exposure to progesterone (P4) prior the timed-AI (TAI) are used to improve the reproductive outcomes. We aimed to evaluate the reproductive performance of prepubertal Nelore (Bos indicus) heifers submitted to different strategies using injectable P4 (iP4, Sincrogest, Ourofino Saúde Animal, Brazil) to induce ovulation prior to the TAI protocol. We hypothesized that Nelore heifers exposed to two iP4 injections could have greater uterus development, contributing to a greater P/AI, lower pregnancy losses and greater calving rate. A total of 1,720 Nelore heifers aged 10 to 17 months were randomly assigned to one of three treatment groups: control group (without ovulation induction prior the TAI; n=576); 1P4 (150 mg of iP4 at D−12 [D0= beginning of TAI protocol]; n=571); and 2P4 (150 mg of iP4 on D−24 and D−12; n=573). The heifers were submitted to an E2/P4-based synchronization protocol and the TAI was performed on D9 by two technicians using thawed semen from four sires. Heifers had the uterus and ovaries assessed on D−24, D−12, and D0 and were attributed scores of 1 (less developed) to 5 (more developed) for both structures. Pregnancy diagnoses were performed on 22, 30-37, 59-61, and 90-120 days after TAI by Doppler and B-mode ultrasound and the calving data was recorded. Data were analyzed by PROC MIXED and GLIMMIX of SAS. The data of 431 pubertal heifers on D−24 were excluded from the analysis. Thus, the control, 1P4, and 2P4 treatment groups were composed of 430, 432, and 427 heifers, respectively. For uterine development, a treatment by time interaction (P <0.05) was observed. In the 2P4 group, uterine development increased progressively between D−24 and D0. In contrast, in the control and 1P4 groups, uterine development increased between D−12 and D0. The 2P4 group [16.2%] presented a greater (P <0.05) CL rate on D0, followed by the 1P4 [8.6%] and control [3.9%] groups. The pregnancy per AI (%) at the four moments of pregnancy diagnoses was greater (P <0.05) in the 2P4 group [37, 32, 27, and 24] than in the control [26, 22, 20, and 17] and 1P4 [27, 25, 21, and 18] groups in all assessments. The calving rate (%) did not differ (P>0.1) among the control [77%], 1P4 [78%] and 2P4 [82%] groups. The pregnancy losses did not differ among the treatment groups (P>0.1). In conclusion, the two administrations of iP4 before the beginning of the TAI protocol contributed to increasing the uterine and ovarian development, and improved pregnancy per AI in prepubertal Nelore heifers submitted to TAI programs. Acknowledgements: Agropecuária Nelore Paranã, Ourofino Saúde Animal, and FAPESP (grant number: 2023/11777-8; and 2024/04783-4)