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 (P(4)) 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 F(2α) (PGF(2α)) on Day 4 Hour 12 (PG1; n = 8), or two treatments (25 mg PGF(2α) 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 PGF(2α) treatment; these cows and one lame cow were excluded. In the 15 remaining cows with physiological interovulatory intervals (18 to 24 d), P(4), 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 PGF(2α) treatment, and then every 2 d, from Day 5 to 8 d after the subsequent ovulation. Because P(4) did not differ (P > 0.05) between PG1 and PG2, cows were regrouped according to their mean P(4) concentration from Days 7 to 15, either P(4) <2 ng/mL (P(4)L; n = 7) or P(4) >2 ng/mL (P(4)H; n = 8). In the treatment cycle, LS was smaller in P(4)L than P(4)H on Days 13 (P = 0.01) and 15 (P = 0.03), and LBF was lower in P(4)L than P(4)H on Day 15 (P = 0.02). The dominant follicle of the first follicular wave was larger in P(4)L than P(4)H 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 P(4)L and P(4)H for P(4), FS, LS, and LBF; however, FBF was lower (P = 0.01) in P(4)L than P(4)H on Day 7. In Group P(4)L, 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 P(4) during diestrus was associated with smaller LS, reduced LBF, and larger FS in the treatment cycle, but not in the subsequent cycle.