Pregnancy outcome is influenced by luteal area during diestrus before successful insemination but not by milk production level

The objective was to compare luteal area (LA), luteal blood flow (LBF), and progesterone (P4) concentration before and after artificial insemination (AI) in pregnant and open cows in the diestrus preceding insemination and during the first 21 d after insemination. A number of 119 multiparous German Holstein cows were examined using B-mode and color Doppler sonography in diestrus (15-5 d before AI) and on days 7, 14, and 21 post insemination (pi). Blood samples for determination of P4 concentration were collected at each examination. In pregnant cows, P4 was measured in the diestrus before AI and on days 7, 14, and 21 of pregnancy, and in open cows, in the diestrus preceding the first AI and 7, 14, and 21 d later. Cows were retrospectively allocated into two groups according to the mean daily milk yield in the first 42 d (high milk yield [HMY], ≥39 kg/d; low milk yield [LMY], <39 kg/d). Pregnant cows had larger LA than open cows during diestrus before insemination (5.69 ± 1.85 vs. 4.94 ± 1.66 cm2, P < 0.05), on days 7 (5.34 ± 1.83 vs. 4.52 ± 1.93 cm2, P < 0.05) and 21 pi (5.92 ± 1.60 vs. 4.97 ± 1.44 cm2, P < 0.05). On day 14 there was a tendency towards larger LA in pregnant cows (P = 0.09). Luteal blood flow (1.72 ± 0.74 vs. 1.22 ± 0.67 cm2) and P4 concentration (8.97 ± 4.37 vs. 6.49 ± 4.32 ng/mL) were higher in pregnant cows than in open cows on day 21 (P < 0.05). At day 150, 69% of the LMY cows and 56% of the HMY cows were pregnant (P > 0.05). Pregnant HMY cows had larger LA during diestrus before insemination and on days 7 and 21 pi than open HMY cows (P < 0.05) and tended to result in larger CLs on day 14 (P = 0.06). Pregnant HMY cows had higher LBF on days 14 and 21 (P < 0.05) and higher P4 concentration on day 21 than open HMY cows (P < 0.05). The main finding of this study was a larger LA during diestrus in cows that conceived in the ensuing estrous cycle compared to cows that did not conceive. Luteal area, LBF, and P4 concentration were greater in pregnant cows compared to open cows, and further studies should be conducted on the usefulness of these variables to detect open cows as early as possible after breeding. High-yielding cows appear to require larger LA, LBF, and higher P4 concentration than low-yielding cows to maintain pregnancy. In conclusion, larger LAs were established in pregnant cows in the diestrus before successful insemination regardless of their milk yield. Further studies are needed to verify whether LA in the diestrus before insemination might be used to predict probabilities in reproductive performance.