Composition of major proteins in cow milk differing in mean protein concentration during the first 155 days of lactation and the influence of season as well as short- term restricted feeding in early and mid-lactation

A variety of proteins contributes greatly to the unique nutritional and functional quality of dairy cow milk. Particularly, milk casein content and composition have substantial influence on the processing capa - bilities. In the present study, milk of 23 multiparous Holstein-Friesian cows, grouped as high- (3.49 ± 0.05%; n = 11) and low-protein (3.03 ± 0.05%; n = 12) cows, was sampled approximately weekly during the first 155 days of lactation to determine the course of relative milk protein composition (α-lactalbumin; β-lactoglobulin; α-, β- , and κ-casein). Furthermore, feed restrictions by 30% of dry matter intake in early and mid-lactation as well as experimental tissue biopsies were conducted to observe their effect on milk protein composition. Milk protein composition was relatively stable and displayed similar concentration patterns throughout the experimental period between both groups. Mean relative concentrations of α-, β-, κ-casein, α-lactalbumin, and β-lactoglobulin were 34.2, 31.4, 16.0, 2.1, and 9.7% of total protein, respectively. Feed restrictions did not alter milk protein composition, whereas the season influenced α- and β-casein as well as α-lactalbumin. Further, effects were observed in both groups at times of unfamiliar stressful situations caused by taking liver or muscle biopsies. As a result, the relative concentration of β-casein increased. Therefore, acute stress factors may lead to a deviation in milk protein composition and should be avoided.