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

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.