(co)variance Components for First Lactation Milk Yield at Three Production and Variability Levels in Argentinean Dairy Cows

INTRODUCTION The discovery of the deep freezing method of cattle semen preservation allowed for a worldwide exchange of genetic material and that the importation of semen from improved breeds have been often used in attempts to increase local livestock productivity. Frequently, this has been done without systematic evaluation of the introduced stocks in the new environment. In 1998, Argentina started a genetic evaluation procedure for yield traits in the Holando Argentino Dairy Cattle population through an animal model. This mixed model genetic evaluation procedure uses the simplifying assumption of equal genetic and residual variances across herds, and that the genetic correlation between genetic expression at different environments is equal to one. However, several studies in which variance components of milk yield were estimated from herds grouped by production and by variability levels have indicated a positive relationship between production level and variability level, and estimates of genetic and residual variances (De Veer and Van Vleck, 1987 ; Boldman and Freeman, 1990 ; Dong and Mao, 1990). The purpose of this work was to estimate (co)variance components for milk yield using a multiple trait model in which yields at three herd production and herd variability levels are considered as three correlated variables. The primary goal was to establish the patterns for variance components and heritability, and to estimate the genetic correlations between genetic expression at the three production and variability levels.