Effects of lactation number, milk yield and milk composition on freezing point of milk of Polish Holstein-Friesian cows

The objective of the study was to assess the influence of lactation number, daily milk yield, somatic cell count and milk composition on the freezing point of milk of Polish Holstein-Friesian cows. The data comprised 3,067,343 test day milk samples collected in 2014 from 865,198 first seven lactations of 714,018 Polish Holstein-Friesian cows, made available by the Polish Federation of Cattle Breeders and Dairy Farmers. The cows calved in 20,043 herds in 2013 and 2014. Four lactation classes were created: first, second, third, and fourth to seventh (4 class). Analysis of variance using the MIXED procedure in SAS was applied to milk freezing point (MFP). The following factors were included in the analysis: lactation class, daily milk yield (16 kg, 16.1–24 kg, >24 kg), fat content (<3.6%, 3.6–4.4%, >4.4%), protein content (<3.1%, 3.1–3.5%, >3.5%), urea content (150 mg·L-1, 151–269 mg·L-1, 270 mg·L-1), lactose content (<4.75%, 4.75–5%, >5%), somatic cell count (<400,000, 400,000) and dry matter in milk (12.5%, 12.6–13.4% and 13.5%), with interactions between lactation and each of the factors mentioned above. MFP decreased with increasing daily milk yield, protein, urea, lactose content and dry matter content, while MFP slightly increased with increasing fat content and somatic cell count. The differences in means of MFP between daily milk yield classes, fat, protein, urea and lactose content, dry matter and somatic cell count were highly significant (P