HETEROGENEITY OF VARIANCE FOR TYPE TRAITS IN THE MONTBELIARDE CATTLE BREED

This paper presents and discusses the estimation of genetic and residu al (co-) variance components for conformation traits recorded in diffe rent environments using mixed linear models. Testing procedures for ge netic parameters (genetic correlations between environments constant o r equal to one, genetic correlation equal to one and constant intra-cl ass correlations, homogeneity of variance-covariance components) are p resented. These hypotheses were described via heteroskedastic univaria te sire models taking into account genotype x environment interaction. An expectation-maximization (EM) algorithm was proposed for calculati ng restricted maximum likelihood (REML) estimates of the residual and genetic components of variances and co-variances. Likelihood ratio tes ts were suggested to assess hypotheses concerning genetic parameters. The procedures presented in the paper were used to analyze and to dete ct sources of variation on conformation traits in the Montbeliarde cat tle breed using 24 301 progeny records of 528 sires. On all variables analyzed, several sources (stage of lactation, classifiers, type of ho using) of heterogeneity of residual and genetic variances were clearly highlighted, but intra-class correlations between environments of typ e traits remained generally constant.