Genetic parameters for a simple predictor of the lifespan of Holstein-Friesian dairy cattle and its relationship to production

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Abstract
The herd life of dairy cows was approximated by a single lifespan value based on the geometric distribution and accounting for the number of lactations each cow had completed or, if still in the herd, was expected to complete. For cows which had completed lactation n, but had not had time to complete lactation n + 1, the probability of survival from lactation n to complete lactation n + 1, from n + 1 to n + 2 etc. was incorporated, utilizing average population survival probabilities. Formulae for the genetic variance in lifespan, its heritability and the genetic regression of breeding value on phenotype, the latter found to be independent of the amount of information included in the phenotype, are derived and checked by simulation. From an analysis of 25227 dairy cows, each with the opportunity to complete five lactations, the heritability of lifespan was 0·06 and its genetic correlation with survival to complete three, four and five lactations ranged from 0·90 to 0·96, for both complete and incomplete survival information, showing that the lifespan trait appears to account adequately for missing observations. Genetic correlations between lifespan and first lactation yield traits were around 0·50, whilst the corresponding phenotypic correlations were approximately 0·13.