Low plasma progesterone concentrations are accompanied by reduced luteal blood flow and increased size of the dominant follicle in dairy cows

To investigate the influence of low plasma progesterone (P4) concentrations on luteal and ovarian follicular development as well as endometrial gene expression in the concomitant and subsequent estrous cycle, 20 lactating dairy (Holstein Friesian and Brown Swiss x Holstein Friesian) cows received either a single treatment with 25 mg prostaglandin F2α (PGF2α) on Day 4 Hour 12 (PG1; n = 8), or two treatments (25 mg PGF2α each) on Day 4 Hours 0 and 12 (PG2; n = 12) of the estrous cycle (Day 1, Hour 0 = ovulation). In four cows, ovulation occurred between 4 and 6 d after the second PGF2α treatment; these cows and one lame cow were excluded. In the 15 remaining cows with physiological interovulatory intervals (18 to 24 d), P4, luteal size (LS) and blood flow (LBF), as well as follicular size (FS) and blood flow (FBF), were determined daily until Day 4, immediately prior to (0 h) and 12 h after each PGF2α treatment, and then every 2 d, from Day 5 to 8 d after the subsequent ovulation. Because P4 did not differ (P > 0.05) between PG1 and PG2, cows were regrouped according to their mean P4 concentration from Days 7 to 15, either P4 <2 ng/mL (P4L; n = 7) or P4 >2 ng/mL (P4H; n = 8). In the treatment cycle, LS was smaller in P4L than P4H on Days 13 (P = 0.01) and 15 (P = 0.03), and LBF was lower in P4L than P4H on Day 15 (P = 0.02). The dominant follicle of the first follicular wave was larger in P4L than P4H on Days 13 (P = 0.03), 15 (P = 0.03), and 17 (P = 0.01). In the subsequent cycle, there were no significant differences between P4L and P4H for P4, FS, LS, and LBF; however, FBF was lower (P = 0.01) in P4L than P4H on Day 7. In Group P4L, endometrial expressions of estrogen receptor α and oxytocin receptor were lower (P = 0.05 and P = 0.03, respectively) at the estrus that preceded treatment compared to the post-treatment estrus. In summary, low P4 during diestrus was associated with smaller LS, reduced LBF, and larger FS in the treatment cycle, but not in the subsequent cycle.