The aim of this study was to assess the influence of calving season on the milk yield and course of lactation of cows fed with Total Mixed Ration system. The study was conducted in herd of 220 cows of Polish Holstein-Friesian breed of Black and White variety (PHF-HO) with an average milk yield exceeding 8 000 kg. It was determined that the calving season influenced the milk yield, its chemical composition and shape of lactation curves. It was demonstrated that calving in the winter months was the least favorable, due to lower productivity in the complete lactation. Births in this season initiated lactations characterized by the lowest daily milk yield in the lactation peak as well as in the last months of lactation. The highest daily milk yield in early lactation, concomitant with the most advantageous milk composition to the 5th-6th month, was associated with autumn calving, which, however, occurred with the lowest frequency.

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