The experiment was performed on a herd of high-yielding Polish Holstein-Friesian (PHF) cows. The animals were housed in a free-stall barn. They were fed a PMR (partially mixed ration) and were milked twice daily using an automatic milking system. The aim of this study was to determine the yield, proximate chemical composition and mineral content of milk from high-producing primiparous and multiparous cows in successive months of 305-day lactation. Milk yield and the concentrations of fat, protein and selected minerals (potassium - K, sodium - Na, calcium - Ca, magnesium - Mg and zinc - Zn) were analyzed in 305-day lactation. In each month of lactation, older cows were characterized by significantly higher productivity – they reached the highest milk production level (peak yield) in the 2nd month of lactation (51.5 kg of milk), and still produced 26.5 kg of milk in the 10th month of lactation. Milk from multiparous cows had higher protein content than milk from primiparous cows. The average protein content of milk from primiparous cows was lowest (3.01%) in the 2nd month of lactation, and it increased to 3.59% in the 10th month of lactation. Milk fat content was higher in primiparous cows, and it increased from 3.94% (peak yield) to 4.75% in the 10th month of lactation. A wide range of changes in the content of the examined mineral components was noted. In each month of lactation in older cows  higher Na and lower Mg content was noted. The low content of Ca in the milk of cows may have been caused by a higher productivity of the analyzed herd. Milk was sampled in successive months after calving during 305-day lactation (360 milk  samples from primiparous cows and 840 milk samples from multiparous cows). Milk samples for chemical analyses of the mineral composition (K, Na, Ca, Mg and Zn) were collected individually from cows during test milking, by the classic method - AT4 (Polish Federation… 2017). Data on daily milk yield and the proximate chemical composition of milk were acquired from RW-2 reports (result report from the milk performance assessment). Proximate chemical composition and the concentrations of selected minerals were analyzed in milk from primiparous (group I) and multiparous (group II) cows. The content of K, Na, Ca, Mg and Zn in milk was determined in analytical samples by wet mineralization with a mixture of nitric acid and hydrochloric acid conducted in a MARS microwave digestion system. Mineralized samples were assayed for the mineral content by atomic emission spectrometry (AES) and atomic absorption spectrometry (AAS). Concentrations of the analyzed minerals in milk were read out from calibration curves.

4.23.2018