GENETIC CHANGES IN PROTEIN, MILK, AND FAT YIELDS AS A RESPONSE TO SELECTION FOR PROTEIN YIELD IN A CLOSED POPULATION OF HOLSTEINS

Three estimators of intrapopulation genetic change were used to quantify response to selection for protein yield in first lactations, of Holsteins from three herds of the Research Branch of Agriculture Canada. Selection began in 1972 and continued through 1985. For the first 3 yr (1972 to 1974), foundation cows were mated by AI to young bulls from the foundation herds and to progeny-tested bulls available in US and Canadian AI units. Progeny-tested bulls produced in this closed herd selection project were not available for selection before 1978. A mixed model analysis based on sire merit revealed an apparent curvilinear response to selection with the most pronounced improvement occurring during the last 5 yr. Sons produced by progeny-tested bulls selected within the project in 1978 and 1979 were superior to the progeny-tested bulls from AI units used in the foundation matings. This sire superiority for protein, milk, and fat yield was 7.3, 193.6, and 7.1 kg, respectively. Genetic trend in the cow population was estimated by two regression procedures. Estimates of annual gain for protein yield were 1.0 and 2.2 kg and protein equivalent to .7 and 1.8% of mean yield. Corresponding estimates of correlated responses were 45.9 and 68.6 kg milk and 1.3 and 2.3 kg milk fat. © 1990, American Dairy Science Association. All rights reserved.