The purpose of this study was to assess the correlation between the sizes of the corpus luteum (CL) with or without a cavity throughout the estrous cycle and progesterone (P4) levels in cows. Images of the CL were obtained by ultrasonography at intervals of 24 h in 10 Holstein cows after their first postpartum estrus. Progesterone levels were measured by radioimmunassay (RIA). The duration of the estrous cycle was determined to be short (13 days), normal (approximately 22 days) and long (30 days) in 1, 7, and 2 cows, respectively. Measurements were evaluated based on the established cycle durations. The size of the CL was measured from the beginning of the second day after ovulation. The CL reached its maximum size on days 6, 9, and 10 in short (SC), normal (NC) and long (LC) cycles, respectively. The P4 levels were below 1 ng/ml on days 0-4 in SC, and 0-2 in NC and LC, and reached a maximum on days 8 (7.31 ng/ml), 10 (9.81 ng/ml) and 18 (6.91 ng/ml) in SC, NC and LC, respectively. The correlation between the size of the CL and the level of P4 was r: 0.72, 0.92, and 0.54 in cows indicating SC, NC, and LC, respectively. In conclusion, CL size and P4 level in cows were correlated. The correlation between the diameter, area, volume and perimeter of the CL was significant.

The purpose of this study was to assess the correlation between the sizes of the corpus luteum (CL) with or without a cavity throughout the estrous cycle and progesterone (P4) levels in cows. Images of the CL were obtained by ultrasonography at intervals of 24 h in 10 Holstein cows after their first postpartum estrus. Progesterone levels were measured by radioimmunassay (RIA). The duration of the estrous cycle was determined to be short (13 days), normal (approximately 22 days) and long (30 days) in 1, 7, and 2 cows, respectively. Measurements were evaluated based on the established cycle durations. The size of the CL was measured from the beginning of the second day after ovulation. The CL reached its maximum size on days 6, 9, and 10 in short (SC), normal (NC) and long (LC) cycles, respectively. The P4 levels were below 1 ng/ml on days 0-4 in SC, and 0-2 in NC and LC, and reached a maximum on days 8 (7.31 ng/ml), 10 (9.81 ng/ml) and 18 (6.91 ng/ml) in SC, NC and LC, respectively. The correlation between the size of the CL and the level of P4 was r: 0.72, 0.92, and 0.54 in cows indicating SC, NC, and LC, respectively. In conclusion, CL size and P4 level in cows were correlated. The correlation between the diameter, area, volume and perimeter of the CL was significant.