Accurate Ultrasonographic Prediction of Progesterone Concentrations Greater than 1 ng/ml in Holstein lactating dairy cows

Contents To develop an ultrasonographic assay for determining plasma progesterone concentration (P 4 ) as < 1 ng/ml or ≥ 1 ng/ml, the corpus luteum (CL) area and P 4 were measured in 1094 multiparous Holstein cows. The area‐measuring function and frozen images were used to outline and measure CL imaged via ultrasonography, and CL area was estimated as a polygon of a continuation straight line. A significant correlation was found between CL area and P 4 (p   <   0.001), and this analysis resulted in the following correlation equation: y = −0.35 + 1.02x ( r  = 0.81). According to the correlation equation, a CL area of 1.3 cm 2 indicated a P 4 of 1 ng/ml. Based on this relationship, each animal was categorized into one of six groups, groups differed based on CL area, and the area ranges were as follows: < 1.3 cm 2 (Group A), 1.3–2.2 cm 2 (Group B), 2.3–3.2 cm 2 (Group C), 3.3–4.2 cm 2 (Group D), 4.3–5.2 cm 2 (Group E) and > 5.2 cm 2 (Group F). For each group, the proportion of cows whose P 4 was 1 ng/ml or more was 1.5% in Group A, 83.3% in Group B, 76.6% in Group C, 96.6% in Group D, 99.2% in Group E and 100% in Group F. There was a significant difference between Group A and the other five groups, and between Groups B or C and Groups D, E or F (p   <   0.005). These results indicate that a functional CL does not exist when the CL area is less than 1.3 cm 2 and that it exists when the CL area is 3.3 cm 2 or more.