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Effect of the Biometrical Relationship among Total Milk Yield, Milk Constituent Yield, and Per Cent of Milk Constituent on Response to Selection¹

Animal Science Department, North Carolina State University, Raleigh

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 per cent 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 two traits and the heritability of either milk yield or per cent milk constituent.

Relative efficiencies of three methods of selecting for either 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. However, 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.

¹ Contribution from the Animal Science Department, North Carolina Agricultural Experiment Station, Raleigh, North Carolina. Published with the approval of the Director of Research as Paper no. 2135 of the Journal Series.