Genetic and Environmental Parameters of Two Field Measures of Milking Speed

Abstract

Single records of 2-min milk yield and total milking time were analyzed including herd-year, age, stage of lactation, and recorded milk yield effects; heritabilities and genetic correlations with production were estimated. Total milking time was measured on 5,083 daughters of 822 sires in 160 herd-years, and 2-min milk was measured on 9,662 daughters of 1,380 sires in 291 herd-years. Herd-years were the most important source of variation and must be considered in rating sires for milking speed. Recorded milk yield, age of cow, and stage of lactation explained significant and decreasing amounts of variation within herd for one or both milking speed traits with 92 and 86% of this variance of 2-min milk and total milking time by linear and quadratic components of the three covariates. Heritability of 2-min milk increased from .15 to .25 when adjusted for recorded milk yield in addition to age and stage. Heritability of total milking time did not change. Genetic correlation between milking speed traits, adjusted for the three covariates, was unity. Total milking time and 2-min milk had negative genetic correlations with milk and fat yield but no correlation with fat percent.