Effect of treatment with long-acting injectable progesterone and/or hCG three days after TAI on circulating progesterone profile, luteal dynamics and fertility of high-producing dairy cows.

The aim was to evaluate the effects of administering long-acting injectable progesterone (iP4) and/or hCG 3 days after timed artificial insemination (TAI) on luteal dynamics, circulating progesterone (P4) concentrations, and fertility outcomes of lactating dairy cows. In experiment 1, synchronized lactating Holstein cows (n = 29) were treated 24 h after induction of luteolysis with 0, 300, 600, or 900 mg of iP4 i.m. to determine circulating P4 profiles over 9 d. In experiment 2, synchronized lactating Holstein cows (n = 75) were allocated in a 2 × 2 factorial design to receive no treatment (Control), 900 mg of iP4 (iP4), 2000 IU hCG (hCG), or both (iP4+hCG) on day 3 of the estrous cycle (day 0 = expected day of endogenous GnRH surge). Ovarian ultrasonography and blood sampling were performed to evaluate corpus luteum (CL) volume and circulating P4 concentrations. In experiment 3, lactating Holstein cows (n = 980) from two farms were synchronized, submitted to TAI on Day 0, and assigned on Day 3 to the same treatments described in experiment 2. Pregnancy diagnosis was performed by ultrasonography on Days 30 and 60 to assess pregnancy per AI (P/AI) and pregnancy loss (PL). In experiment 1, iP4 increased circulating P4 in a dose-dependent manner, with the 900 mg dose producing the highest (P < 0.01) mean P4 concentrations (1.8 ± 0.2 ng/mL) over 9 d. In experiment 2, no iP4 × hCG interaction was observed. The iP4 treatment increased (P < 0.01) circulating P4 on Days 5 and 7 without affecting CL development (P = 0.45), while hCG increased CL volume on Days 9 and 13 (P < 0.01), and P4 concentrations on Day 9 (P = 0.05). In experiment 3, there was no effect of hCG nor interaction iP4 × hCG on fertility outcomes. However, iP4 increased overall P/AI at 30 days (34.3 [170/495] vs. 26.6% [129/485]; P < 0.01), with effects depending on farm and season. The beneficial effect of iP4 on fertility outcomes was observed only on one of the farms, and only during the hot season. In conclusion, hCG treatment 3 days after TAI transiently increased CL volume and P4 concentrations but did not improve fertility. Conversely, iP4 supplementation increased circulating P4 without impairing CL function and improved fertility of lactating dairy cows, although this benefit appeared conditional on environmental and management factors.