The aim of the research was to analyse changes in milk fat fraction content depending on the ration. The research was conducted on 155 cows in the first half of lactation. Milk samples were collected during two consecutive periods of summer and winter feeding. In total 310 milk samples were collected and then components of milk fat fraction were analysed: functional fatty acids (butiric - BA, oleic - OA, transvacenic - TVA, linoleic - LA, conjugated linoleic acid - CLA, linolenic - LNA, eikozapentaenic - EPA, dokozapentaenoic - DPA and dokozaheksaenoic - DHA), vitamins A, E and carotene, cholesterol and free fatty acids (FFA). Fatty acid analyses were performed by the gas chromatography method, vitamins were determined using a UV-VIS spectrophotometer, and cholesterol and free fatty acids by thin-layer chromatography TLC as well as a UV-VIS spectrophotometer. Moreover 33 feeding stuff samples were collected and analysed (dry matter, total protein, crude fat, crude fibre) and then their dietary value was determined. The results obtained indicate that in the case of a summer ration, when cows are fed diet with pasture, milk produced contains more of functional fatty acids: OA, TVA, CLA, LNA, EPA, DHA, and vitamins A, E and carotene than milk from cows fed conserved feedstuff. BA, whose content in milk is higher during feeding with a winter ration, constitutes an exception.
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