Length of Productive Life of Dairy Cows. 2. Variance Component Estimation and Sire Evaluation

Abstract

Abstract Estimates of the genetic component of the cow's true stayability and of an approximation of her functional stayability were computed from length of productive life records of 83,338 grade Holstein daughters of 2182 sires. True stayability is defined as the aptitude of a cow to delay culling, whereas functional stayability refers to the ability to delay involuntary culling. The probability of a cow being culled is described using two Weibull models with time-dependent covariates. The first one includes fixed herd × year and stage of lactation × lactation number effects and a random sire effect. In the second model, a within herd × level of milk production is included in order to correct the sire effects for the major source of voluntary disposal. Such a model, actually characterizing “milk-corrected” stayability, is a first step toward a functional stayability evaluation. The empirical Bayes estimates of the sire variance component are very similar for both models. Rigorously speaking, phenotypic variance of length of productive life cannot be computed because of the presence of time-dependent covariates in the models. If this problem is ignored, a “pseudoheritability” of 8.5% is obtained for both traits. The correlation between the two sire evaluations is only .74. Milk yield is favorably related to true stayability but slightly opposed to milk-corrected stayability. Functional stayability is presented as an appealing and economically important secondary trait to consider in breeding programs as a complement to the current production traits.