GENETIC-PARAMETERS FOR A SIMPLE PREDICTOR OF THE LIFE-SPAN OF HOLSTEIN-FRIESIAN DAIRY-CATTLE AND ITS RELATIONSHIP TO PRODUCTION

The herd life of dairy cows was approximated by a single lifespan valu e based on the geometric distribution and accounting for the number of lactations each cow had completed or, if still in the herd, was expec ted to complete. For cows which had completed lactation n, but had not had time to complete lactation n + 1, the probability of survival fro m lactation n to complete lactation n + 1, from n + 1 to n + 2 etc. wa s incorporated, utilizing average population survival probabilities. F ormulae for the genetic variance in lifespan, its heritability and the genetic regression of breeding value on phenotype, the latter found t o be independent of the amount of information included in the phenotyp e, are derived and checked by simulation. From an analysis of 25227 da iry cows, each with the opportunity to complete five lactations, the h eritability of lifespan was 0.06 and its genetic correlation with surv ival to complete three, four and five lactations ranged from 0.90 to 0 .96, for both complete and incomplete survival information, showing th at the lifespan trait appears to account adequately for missing observ ations. Genetic correlations between lifespan and first lactation yiel d traits were around 0.50, whilst the corresponding phenotypic correla tions were approximately 0.13.