Bayesian estimation of genetic parameters for test day records in dairy cattle using linear hierarchical models

Hierarchical models were fitted to first lactation test day milk yields of Canadian Holstein cows. Models included Wilmink's lactation curve and herd- test date effects in the first stage, and herd-year-season of calving, regi on-age-season of calving and animal genetic effects in the second stage of the hierarchy, using various combinations of these factors. Functions of Wi lmink's function trajectory parameters were analysed jointly with the origi nal parameters in selected models. Genetic and environmental variances, and heritabilities for lactation curve coefficients, daily and 305-day yields, and persistency of lactation were inferred by Bayesian methodology via Gib bs sampling. Accounting for the between herd-year-season variation in traje ctory parameters resulted in more stable estimates of genetic variances ove r lactation, more sensible heritabilities for extreme days in milk, larger genetic correlations between yields at distant days of lactation, but lower heritabilities of lactation curve coefficients. Joint modelling of lactati on curve parameters and selected functions thereof allowed for estimation o f heritabilities of time-dependent traits, and reduced seemingly anomalous estimates of genetic parameters obtained with current implementations of ce rtain random regression models. (C) 2001 Elsevier Science B.V. All rights r eserved.