Use of injectable progesterone to replace the intravaginal progesterone device on the ovulation synchronization protocol reduces the pregnancy rate in Bos indicus cows

The aim of this study was to evaluate the effectiveness of using different doses of injectable progesterone (P4i) in a new ovulation synchronization protocol in place of progesterone (P4)-releasing intravaginal devices in Bos indicus cows. For this, three experiments were carried out. To determine the P4-release curve (Experiment 1), 55 Bos indicus cows were distributed into 5 experimental groups for the admin-istration of different doses of P4i at D0 (P4i60, n = 11; P4i105, n = 11; P4i150, n = 11; P4i195, n = 11 and P4i240, n = 11) and submitted to the ovulation synchronization protocol (D0: P4i + EB; D8: PGF2a + EC + eCG). Daily blood samples were collected by jugular venipuncture from D0 to D12 for analysis of the serum P4 profile. To evaluate ovarian follicular dynamics and the timing of ovulation (Experiment 2), three studies were conducted with different doses of P4i at the beginning of the ovulation synchroni-zation protocol (Study a: 150 mg and 105 mg P4i; Study b: 75 mg P4i; Study c: 60 mg P4i). At the end of the ovulation synchronization protocol, after ovulation inducing administration, ultrasound examina-tions were performed every 24 h until 96 h or until ovulation was detected (Studies a, b and c). To evaluate the pregnancy rate (Experiment 3), 132 Bos indicus cows were subjected to an ovulation syn-chronization protocol using either a 75 mg progesterone injection (Group P4i75) or an intravaginal P4 device (control group) on D0. Pregnancy was determined by ultrasound 30 days after the end of the ovulation synchronization protocol. All statistical analyses were performed by SAS (R). In the P4-release curve, the peak occurred on D1 and on D3 all concentrations remained below 1 ng/mL until the end of the study (D12) in all groups. The ovulation rates were similar between the 75 mg dose group and the Control group (P = 0.24 -Experiment 2 -study b), result not observed with other doses of P4i (study a and c). The pregnancy rate (Experiment 3) was greater [P4i Group 27% and Control Group 72.7% (P = 0.0001)] in cows receiving the intravaginal P4 device. In conclusion, the use of long-acting P4i (75 mg) to replace the intravaginal P4 device negatively affects the conception rate of Bos indicus cows submitted to ovulation synchronization protocol.(c) 2022 Elsevier Inc. All rights reserved.