Effect of progesterone supplementation during the first follicular wave and delaying induced luteolysis with a single prostaglandin F2α on time of luteolysis and ovulation in Holstein heifers.

This study aimed to evaluate the effect of progesterone (P4) supplementation during the first follicular wave and delayed induced luteolysis with only one PGF 2 (PGF) treatment on d 6 of the modified 5d-CoSynch protocol on ovarian dynamics, serum P4 concentrations, and estrus expression of Holstein heifers.Heifers were pre-synchronized with a PGF on experimental d -2 (pre-PGF) and treated with GnRH on d 0 to induce a first follicular wave of the estrous cycle.On d 0, heifers were randomly assigned (n = 34/treatment) to one of 3 treatments (G5PP-P4, G5PP and G6P).Heifers in the G5PP-P4 had an P4 intravaginal insert placed on d 0 that was removed on d 5. Heifers in G5PP and G5PP-P4 received 2 PGF on d 5 and d 6 (PGF-1 and PGF-2, respectively).Heifers in the G6P received only one PGF on d 6 (PGF-1).Only heifers that had luteolysis and ovulation between pre-PGF and 2 d after GnRH were considered in the first follicular wave and used in the study (G5PP-P4, n = 25; G5PP, n = 29; and G6P, n = 25).Ovarian ultrasound was performed on d -2, 0, 2, 3, at the time of PGF-1 (d 5 for G5PP-P4 and G5PP, and d 6 for G6P), 24h after PGF-1, and then every 12h until ovulation.Additionally, ovarian ultrasound was also performed on d 5 in heifers in the G6P group.Blood was collected on d -2, every day from GnRH until PGF-1, and every 12h after PGF-1 until ovulation or 144h after PGF-1.Heifers supplemented with P4 had greater serum P4 from d 1 to d 5 than heifers not supplemented.Heifers in G5PP-P4 had slower follicular growth and smaller dominant follicles (DF) at PGF-1, and G6P heifers had the largest DF at PGF-1.By 36 and 48 h after PGF-1, all heifers in G5PP-P4 and G5PP, respectively, had serum P4 0.45 ng/mL, whereas only 80% of heifers in G6P reached this threshold by 48 h.Treatment did not affect the proportion of heifers with functional CL at PGF-1, expressing estrus, and ovulating after PGF-1.However, despite a shorter interval from PGF-1 to ovulation, the ovulatory follicle was older and larger in G6P heifers at PGF-1 compared with those in the other treatments.In summary, not supplementing P4 and administering only one PGF on d 6 of the first follicular wave of dairy heifers altered the ovulation time but resulted in a similar proportion of heifers with ovulation at the end of the protocol.However, our results suggest that a single PGF dose on d 6 after GnRH may not be as efficient in promoting luteolysis as 2 PGF administrations on d 5 and 6.The impact of adopting these changes on fertility in a timed-AI protocol for dairy heifers such as a 5d-CoSynch still requires further investigation.