Theoretical Basis for Genetic Parameters of Herd Life and Effects on Response to Selection

A theoretical model of herd life was used to quantify biases in genetic parameter estimates from culling on production and to study effects on response to selection. Herd life was modeled as a linear function of production and ability to survive regardless of production (survival). Genetic improvement of survival is of interest. Results of analytically derived fonnulas showed that estimates of heritability obtained from analysis of herd life are biased for survival. For moderately negative genetic correlations between production and survival. biases tended to be upward. The genetic correlation between production and survival was severely overestimated when based on production. and herd life. Adjustment of phenotypic herd life for phenotypic production removed some of the biases in genetic parameter estimates unless little direct culling for production occurred.