The increase of milk production observed in dairy cows after calving is usually related to insufficient feed intake, which leads to negative energy balance (Drackley, 1999a, b). The energy demand of high-yielding cows in the peak of lactation exceeds the amounts of net energy from diet. This is a result of a decreased appetite after parturition and an increase in metabolic processes, which in consequences requires large amounts of substrates for milk constituents synthesis. A lack of sufficient nutritional components in the feed results in the use of body reserves, because in order to decrease the energetic deficiency, surplus fat is mobilized and undergoes lipolytic processes. Fat hydrolysis causes increased fatty acids metabolism in the liver, resulting in adipose tissue degeneration (Mulligan et al., 2006). Intensified lipolysis leads to metabolic disorders like hyperketonemia, hypoglycemia, and hyperbilirubinemia (Ingvarsten et al., 2003;Mulligan et al., 2006). These metabolic disorders negatively affect the yield, chemical composition, and technological parameters of milk (Rzewuska et al., 2011) and fertility traits as well (Roxtrom et al., 2000). Under conditions of energy deficiency also rumen functional disorders can be noticed. The function of the digestive tract organs determines the metabolic balance of the whole organism of a high yielding cow (Ingvarsten et al., 2003;
Relationship between milk yield, stage of lactation, and some blood serum metabolic parameters of dairy cowsA. Jóźwik, N. Strzałkowska, E. Bagnicka, W. Grzybek, J. Krzyżewski, E. Poławska, A. Kołataj, J.O. HorbańczukInstitute of Genetics and Animal Breeding PAS in Jastrzębiec, Wólka Kosowska near Warsaw, Poland ABSTRACT: 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 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.