Ultrasonographic appearance of tissue is a better indicator of CL function than CL diameter measurement in dairy cows

In this study, the ovaries of 99 randomly selected Friesian cows were examined by ultrasonography measuring the diameter and evaluating the appearance of corpora lutea (CLs) in order to assess the most reliable method for their functional classification. Concurrently, blood samples were taken and analyzed for plasma progesterone (P4) concentration. On the basis of the ultrasonographic measurement of the diameter of the CL, three groups were established: (A) CL not detected (n = 30), (B) CL psi < 20 mm (n = 22), and (C) CL psi > or = 20mm (n = 47). On the basis of the ultrasonographic appearance, three different groups were established: (A) CL not detected (n = 30), (B) evolving CL (n = 25), and (C) mid-cycle CL (n = 44). On the basis of the P4 values, CLs were functionally classified in the following three groups: (A) CL not detected when plasma P4 was lower than 1 ng/ml (n = 27), (B) evolving CL when plasma P4 was between 1 and 4 ng/ml inclusive (n = 29), and (C) mid-cycle CL when plasma P4 was more than 4 ng/ml (n = 43). The degree of agreement between plasma P4 concentrations and either ultrasonographic classification (diameter or appearance) was highly significant (P < 0.001). However, the results of the present study suggest that for the evaluation of functional classification of the CL in cows ultrasonographic appearance is more reliable than the evaluation of the diameter.