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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 experi- ment 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 60 th and the 200 th 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.

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