An evaluation for cow productivity traits of British Holstein-Friesian cattle was carried out on 10314 first lacta-tion records including 228 tested sires. Yields of 305-day milk, fat and protein were the traits examined. Restricted Maximum-Likelihood procedure was used to estimate va-riance and covariance components between and within sires. Analysis was carried out using a mixed model with a herd-year-season as a fixed effect and sires as a random effect. Heritabilities were 0.25, 0.24 and 0.23 for milk, fat and protein yields, respectively. Genetic and phenotypic correla-tions were positive and of high magnitude and ranged from 0.76 to 0.94. Single-trait selection for milk yield resulted in an expected increase of 4.4, 3.2 and 3.4 as a percentage of the overall mean of milk, fat and protein yields, respectively. Selection for fat yield compared with selection for milk yield resulted in —1.12, +0.93 and —0.29% of milk, fat and protein, respectively, while selection for protein yield re-sulted in —0.07, 0.0 and -1,-0.29%. Therefore, selection for milk yield is more economic than selection for fat or for protein yield. Five indices of selection for improving yield traits of cows were constructed involving all combinations of two or three traits studied. Fat or protein yields did not contribute significantly to the different selection indices constructed while milk yield made a large contribution. Con-siderable genetic improvement for yield traits of cows might be achieved through multiple-trait selection based on reduced index including milk yield with either of fat yield or protein yield.
|
