Relationships between Distribution of Major Milk Proteins and Milk Yield

Daily yield and composition of casein and serum proteins of milks from Holstein and Jersey cows were studied during two 9-day periods of lactation to determine whether selection for milk yield influences distribution of total protein between casein and serum fractions. Casein number, defined as casein/total protein as percent, was the distribution criterion. Casein number of Holsteins was not associated with daily or mature equivalent yield of milk, estimated real producing ability, or United States Department of Agriculture Cow Index for milk yield. In early lactation, Jerseys had 4.7% more protein as casein than Holsteins. The intralactation repeatability of casein number for Holsteins was .31. Standard deviation of casein number due to cow was 2.3 and was worth $29.60/ 305-day lactation yielding 7483 kg milk.