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Parameter Estimation of Milk Yield and Composition for 305 Days and Peak Production

Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24060

S. Korver and J.A.M. Van Arendonk

Department of Animal Breeding, Wageningen Agriculture University, PO Box 338, 6700 AH Wageningen, The Netherlands

ABSTRACT

Test-day data of milk, fat, and protein yields of first lactation Black and White cows calving from 1981 to 1985 in The Netherlands were used to calculate production variables for the lactation periods of 40 to 100 d and 0 to 305 d. For each lactation period, genetic parameters were estimated for the variables, milk yield, fat yield, protein yield, fat plus protein yield, ratio of protein to fat, fat percent, and protein percent using multivariate REML analysis. Heritabilities of fat percent, protein percent, and the ratio of protein to fat were considerably smaller from 40 to 100 d (.40, .47, .31) than from the entire lactation (.74, .62, .69). The genetic correlation between milk yield and the ratio of protein to fat was positive (.19) for the entire lactation but negative (–.15) for the 40 to 100-d period. Genetic correlations between die 305-d trait and its corresponding 40 to 100-d trait were all larger than .90.