Date of Award
Doctor of Philosophy
A. E. Freeman
First lactation milk, fat, protein, and lactose yields and percentage yields were analyzed using a multiple-trait sire model including herd-year-season, sire group, and age of cow as fixed effects. Variance components were estimated using restricted maximum likelihood with an expectation-maximization algorithm, and included sire relationships. Somatic cell score (SCS) was fit both as a fixed effect in the model and as an additional dependent variable in two analyses. The results were almost identical for both analyses. Lactose percentage means ranged from 4.84 to 4.97% across three dairy breeds. Data used to estimate variance components were first lactation Holstein records from 5246 daughters of 392 AI sires, collected from 1986 to 1988. Heritability estimates were.30,.29,.27, and.26 for milk, fat, protein, and lactose yields,.45,.47, and.53 for percentage yields, and.16 for SCS, respectively. Genetic correlations of lactose percentage with milk, fat, protein, fat and protein percentages, and SCS were -.30, -.16, -.21,.16,.29, and -.11, respectively, while phenotypic correlations were -.08, -.02,.01,.11,.29, and -.15;A gene flow algorithm developed by Harris and Freeman (1991), which accounted for multiple stage selection with overlapping generations, and the previous estimates of variance components were used to estimate response to selection. Progress could be made in any single trait through single-trait selection for that trait. Desirable genetic gains in all economically important traits, however, could not be achieved simultaneously by single-trait selection for any one trait. Multiple-trait selection indexes involving milk and protein yields would be expected to result in genetic gains similar to single-trait selection for milk and protein yields. In addition to the increase in milk and protein yields, lactose percentage would decrease and fat percentage would increase minimally, but somatic cell score would increase.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Robert Donald Welper
Welper, Robert Donald, "Estimation of variance components and response to selection for milk composition in Holsteins, including lactose and somatic cell score " (1991). Retrospective Theses and Dissertations. 10087.