Adjustments to Cow Indexes for Milk Yield for Effects of Herd Mean and Standard Deviation

Abstract

Lactation records of cows born since 1964 were used to compute Modified Contemporary Comparison cow indexes for 563,853 Holsteins and 339,568 Jerseys. Eight adjustments were made to cow indexes according to herd mean and standard deviation: two varied heritability to adjust for genetic effects, two adjusted Modified Contemporary Deviation for within-herd environmental variation, and four were combinations of these. Rankings of cow indexes for Holsteins, based on R2, differed considerably for predicting performance of four offspring groups (all daughters, daughters of elite cows, all sons, and artificial insemination sons). Rankings of cow indexes for Jerseys were more similar across offspring groups. Under the assumption that large true breed differences are unlikely, the best cow index for both breeds had Modified Contemporary Deviation standardized to a common variance with constant heritability. This cow index adjusted for environmental effects of herd variance, ignoring differences in genetic variation. It decreased differences among cow indexes for cows in high variance herds, and increased differences in low variance herds.