Effect of the Biometrical Relationship among Total Milk Yield, Milk Constituent Yield, and Per Cent of Milk Constituent on Response to Selection

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Abstract
The biometrical interrelationships among total milk yield, milk constituent yield, and per cent of milk constituent are considered. Formulas are developed which express the parameters of percent constituent in terms of the parameters of total milk yield and milk constituent yield. Genetic and phenotypic statistics calculated directly from dairy cattle data agree with those obtained by application of the formulas. Formulas are presented for genetic gain in milk constituent yield by indirect selection on either total milk yield or per cent of milk constituent. These formulas show that genetic gains in milk constituent yield by indirect selection are determined by the means and genetic standard deviations of total milk yield and per cent constituent, the genetic correlation between these 2 traits and the heritability of either milk yield or per cent milk constituent. Relative efficiencies of 3 methods of selecting for either solids not fat [SNF] yield or fat yield are compared in the population studied. Selecting for SNF yield using total milk yield was slightly less efficient than direct selection. The relative efficiency of indirect selection using per cent SNF was considerably lower. Loss in efficiency of indirect selection for fat yield, using total milk yield, is not greatly different from that using per cent fat.