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Influence of lactation stage on selected blood parameters and biological value of cow milk during pasture season in organic system of production. The disproportion between the cow's genetically determined ability to produce milk and the limits to improving the energy value of feed may be the cause of the metabolic stress of animals kept at organic farms. Negative energy balance has a major impact on the body's hormonal balance and the function of the organs; it also affects the content of basic components (protein, fat and lactose) in cows' milk. The aim of the study was to investigate changes in the biological value of organic milk obtained from high producing dairy cows at different stages of lactation during pasture season. The study was carried out on 68 Polish Holstein-Friesian primiparous. 272 milk samples were collected from cows which were in one of the following phases: the beginning of lactation (BL): 5-30 days postpartum); peak of lactation (PL): 60-90 days postpartum; full lactation (FL): 120-150 days postpartum); end/final phase of lactation (EL): 250 days postpartum. Stage of lactation had an overriding effect on the majority of analyzed milk constituents including: whey proteins, vitamins soluble in fat, phospholipids and fatty acids. The highest levels of lactoferin were found at the beginning of lactation (BL). At the same time, the β-hydroxybutyrate (BHBA) concentration increased, which confirmed that the fast growing milk yield at the peak of lactation is linked to an increase in energy demand. The lowest alanine aminotransferase and non esterified fatty acids activity was measured during BL. The study has shown a significant influence of lactation stage on the biological value of organic milk obtained from high producing dairy cows during pasture season. In addition, there is evidence that the organic production system has no influence on the metabolic profile of cows as the plasma parameters were within their standard norms. This might be the result of the positive effect of grass antioxidants.
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