Variation of milk protein profile of cows from multi-breed herds

The aim of present work was to study the variation of milk protein fractions (αs1-CN, αs2-CN, β-CN, κ-CN, β-LG, α-LA), expressed as g/L of milk, and as percentage of the total nitrogen (%N) of milk, in relation to the breed, parity, and days in milk (DIM) of the cow, and of CSN2, CSN3, BLG genotypes. A total of 1500 dairy cows belonging to 6 breeds (3 specialised dairy breeds: Holstein-Friesian, Brown Swiss, Jersey; and 3 dual-purpose breeds: Simmental, Alpine Grey, Rendena) were sampled from 41 multi-breed herds (from 2 to 5 breeds per herd) located in Trentino-Alto Adige region (Northeast Italy). High- Performance Liquid Chromatography (HPLC) permitted the quantification of all the major protein fractions and the identification of β-CN, κ-CN and β-LG genetic variants. Protein fractions were analysed using a linear mixed model, which considered breed, parity, DIM, and CSN2, CSN3, BLG genotypes as fixed effects, herd as random effect and herd productivity as covariate. Results showed that parity and DIM influenced all protein fractions, both expressed as g/L and %N. Also, the genotypes of CSN2, CSN3 and BLG affected almost all the traits considered. Even after correcting for the effects of the protein genotypes, breed of cows showed large differences: Holstein-Friesian cows had the lowest milk protein content, but with the highest proportion of true protein, mainly because of a high proportion of β-CN and whey protein; Jersey cows had the highest milk protein, casein and particularly αs1-CN, contents, but the lowest proportion of true protein and content of whey proteins; Brown Swiss and Simmental cows showed intermediate results; Alpine Grey cows were characterised by the highest proportion of true protein and whey protein (like Holsteins); lastly, Rendena cows showed the highest proportion of casein (like Jerseys), content of β-CN, as Holstein- Friesian, and proportion of αs2-CN and κ-CN. This study allowed to disentangle the effect of 6 dairy and dual-purpose breeds due to the major genes of CSN2, CSN3 and BLG from those due to the other genes, in the same herds and taking into account the effect of herd productivity.