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Improving an Aggregate Phenotype in Replacements by Selecting Their Sires Conditioned on Dam Phenotypes¹

Department of Dairy Science, The Ohio State University, Columbus 43210

G. E. Shook

Department of Dairy Science, University of Wisconsin, Madison 53706

L. P. Johnson

Holstein-Friesian Association of America, Brattleboro, VT 05301

ABSTRACT

An indexing of total merit in expected progeny is outlined when the constituent merit from each trait is defined as a discrete quadratic equation. Information consisting of an estimated breeding value for each trait on each potential parent is used to define a trait index or a trait merit index for progeny. The selection of a specific sire for each mating, vis-a-vis the selection of a high ranking sire independent of any particular mating, is indicated more frequently when an interval along a trait scale exhibits strongly curvilinear merit and also spans the variation in trait index values among alternative matings. Variability in a trait index increases as heritability increases. Nonlinearity in a merit function dynamically describes the direction and relative selection pressure from one trait relative to another when indexes depend on dam or herd mean phenotypes. A method is proposed to quantify the role of merit from each trait relative to total merit. An example using three traits, milk yield, udder depth score, and rear leg score, is illustrated and evaluated.

¹ Contribution to NC-2, Improving Dairy Cattle, with special emphasis on selection.

² Salaries and research support provided partially by State and Federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. Journal Article No. 118-85.

³ Present address: University of New England, Animal Genetics and Breeding Unit, Armidale, Australia.