The objective of this experiment was to determine the effect of high versus low progesterone (P4) during the pre-dominance or dominance phase (or both) of ovulatory follicle development on follicular dynamics and fertility of lactating dairy cows. Progesterone (P4) was manipulated to reach high (H) or low (L) serum concentrations during the pre-dominance phase (d 0 to 4 of the wave) and dominance phase (d 5 to 7 of the wave) of a second follicular wave ovulatory follicle, creating 4 treatments: H/H, H/L, L/H, and L/L. Luteolysis was induced with PGF2α on d 7 of the wave and ovulation was induced with GnRH 56 h after PGF2α. Cows (n = 558) received artificial insemination (AI) 16 h following GnRH. Pregnancy was determined at 6 intervals during gestation and at calving to quantify pregnancy loss beginning at d 23 post-AI utilizing pregnancy-specific protein B (PSPB) in novel within-cow comparisons. Cows with single ovulations assigned to the L/L treatment had greater pre-ovulatory follicle diameter compared with cows assigned to the L/H or H/L treatments. Cows with single ovulations had greater pre-ovulatory follicle diameter compared with cows with double ovulations. Low P4 in H/L, L/H, and L/L increased double ovulation rate compared with H/H. Cows with double ovulations had greater pregnancies per AI (P/AI) on d 23 post-AI compared with cows with single ovulations but had greater losses if ovulations were unilateral. Cows with low P4 during the entire period of the ovulatory follicle development also had greater P/AI on d 23 post-AI compared with cows with high P4 during both phases. However, full-term P/AI was not different between treatments. This was a result of the greater incidence of pregnancy losses between d 35 and 56 of gestation for cows with unilateral double ovulations compared with bilateral double ovulations and single ovulatory cows. Cows with single ovulation and low circulating P4 during the dominance period of follicle development had increased pregnancy losses between d 35 and 56 of gestation compared with cows with single ovulations and high P4. The PSPB measurements on d 16 and 23 post-AI were highly accurate in the prediction of pregnancy at d 28. The PSPB differed on d 23 and 28 between cows that had versus cows that did not have pregnancy losses between d 28 and 35 of gestation. In summary, circulating concentrations of P4 during ovulatory follicle development affected numbers of follicles ovulated and timing of subsequent pregnancy losses.

The objective of this study was to determine the effect of high vs. low circulating concentrations of progesterone (P4) during early and late stages of ovulatory follicular wave development on ovulatory follicle size and double ovulation rate in lactating dairy cows. Lactating dairy cows were synchronized with prostaglandin F2α (PG; 25 mg of dinoprost tromethamine, i.m.) then gonadotropin releasing hormone (GnRH; 100 µg of gonadorelin; i.m.) 2 d apart to initiate a new cycle. Only cows that started a new cycle were used in this study. On d 7 of the estrous cycle, all cows received GnRH (200 µg; i.m.), were blocked by parity and AI service number and then randomly assigned to four groups. Only animals that ovulated to this 2nd GnRH injection were used in the study. The day of this GnRH injection was considered d0 of the ovulatory follicular wave and treatment. PG and/or CIDRs were utilized to induce high (H) or low (L) circulating concentrations of P4 during either 0 to 4 d (early) and/or 5 to 7 d (late) of the ovulatory wave in a 2 x 2 randomized design, H/H, L/L, L/H and H/L. All cows (n=471) received PG 7 d after initiation of treatment. One d later, CIDRs were removed and a second PG was administered to ensure complete CL regression. GnRH was administered to all cows 56 h after the first of these two PGs to induce ovulation. Ovaries were evaluated by transrectal ultrasonography at time of GnRH administration and 2d later to determine size and number of ovulatory follicles. Variables were analyzed using the GLIMMIX procedure of SAS. There was an effect of treatment on mean diameter of single ovulating follicles (P < 0.001) and percentage of cows with double ovulations (P < 0.001). Mean diameter of single ovulating follicles was smaller for HH (16.8 ± 0.2 mm) compared to HL, LH and LL treatments (18.2 ± 0.3; 17.8 ± 0.2; 19.6 ± 0.3 mm, respectively; P < 0.02). HL and LH had smaller single ovulating follicles compared to LL (P < 0.001). Single ovulating follicles tended to be smaller in LH compared to HL (P = 0.09). Percentage of cows with double ovulations was greater in LL (51%) compared to HH, HL and LH (13, 34, and 32 %, respectively; P < 0.01). Cows in HH had lower rates of double ovulation compared to HL and LH (P < 0.001). An interaction between parity and service number for double ovulation rate was significant (P < 0.02) and determined that primiparous first service cows had lower double ovulation rates compared to primiparous later service cows and multiparous first and later service cows (14 vs. 36, 35, and 40 %, respectively; P < 0.001). In summary, these data indicate that serum concentrations of P4 during specific stages of the follicular wave directly affect size and number of ovulating follicles.