Relationship between milk yield, stage of lactation, and some blood serum metabolic parameters of dairy cows

The aim of the study was to evaluate the effect of milk yield and stage of lactation on the activity of liver enzymes, cholesterol, and vitamin C concentration in blood of milking cows. The experiment was carried out on Polish Holstein-Friesian Black and White dairy cows with two different milk yield levels: M - medium (about 7000 kg per lactation) and H - high (about 10 000 kg per lactation). In blood serum, AST, ALT, GGT, CHOL, and vitamin C were estimated. The AST and ALT activities in the blood serum were lower in M group than in H group, however within M and H groups there were no differences in both aminotransferases activity between the 60th and the 200th day of lactation. Differences in GGT activity (P <= 0.01), CHOL (P <= 0.05), and vitamin C level (P <= 0.01) in blood serum were found between both stages of lactation. Negative correlations between vitamin C level with somatic cell count and milk yield traits were observed, that may indicate an increase in oxidative processes in high-yielding dairy cows. The achieved results may be used in diagnostics and/or evaluation of herds from the point of view of biochemical and pathophysiological processes.