Phenotypic and genetic statistics of components of milk and two measures of somatic cell concentration.

Heritabilities, genotypic and phenotypic correlations for milk, fat, and protein yields, and two traits related to somatic cell concentration (cumulative lactation score and lactational somatic cell concentration) were estimated. A total of 18,416 first lactations of Holstein cows were analyzed by a new procedure for estimating variance components. Heritabilities were .21, .23, .19, .17, and .61 for milk, fat, and protein yields, cumulative lactation score, and lactational somatic cell concentration. Addition of protein yields to the current selection for two traits with nil economic value for protein would improve genetic gains for fat and milk yields in the northeastern United States. If cumulative lactation score and lactational somatic cell concentration were incorporated in current selection for two traits, restricted selection indexes should be used to avoid reduction in genetic gains for milk and fat yields.