Restricted maximum likelihood estimation of genetic parameters for the first three lactations in the Montbéliarde dairy cattle breed

of genetic parameters for the first three lactations in the Montbliarde dairy cattle breed C. Beaumont 0 0 Institut National de la Recherche Agronomique, Station de Recherches Avicoles , Nouzilly, 37380 Monnaie , France Summary - Genetic parameters for the first three lactations have been estimated for the main dairy traits (milk, fat, protein and useful yields adjusted for lactation length, fat and protein contents). Two data sets were analysed, including records on 30 751 cows born from 128 young sires and 52 proven sires. Daughters' performances from the most widely used proven sires were incorporated in order to improve the degree of connectedness among herds. The model fitted young sires as random and proven sires, herd-year, seasonyear of calving, age at first calving and length of the previous lactation as fixed effects. Relationships among bulls were included. Analysis was by restricted maximum likelihood using an EM-related algorithm and a Cholesky transformation. All genetic correlations were larger than 0.89. Correlations between the first and third lactations were slightly lower than the others. Heritabilities of milk, fat, protein and useful yields ranged from 0.17 to 0.27. Phenotypic correlations between successive lactations were higher than 0.6 and those between lactations 1 and 3 lower than 0.55. Heritabilities of fat and protein contents were higher than 0.44 with phenotypic correlations being stable at about 0.70. The "repeatability model" which considers all lactation records as a single trait can be considered in genetic evaluation procedures for dairy traits without significant losses in efficiency. dairy cattle - milk yield - fat and protein contents - genetic parameters - maximum likelihood Rsum - Application de la mthode du maximum de vraisemblance restreint (REML) l'estimation des paramtres gntiques des trois premires lactations en race montbliarde. Ce travail a pour =but l'estimation des paramtres gntiques des3 premires lactations des femelles Montbliardes et porte sur les principales caractristiques laitires (productions, ajustes pour la dure de lactation, de lait et de matires utiles, grasses et protiques,, tnux butyreux et protique). Deux fichiers sont tudis. Ils rassemblent les performances de 30 751 femelles issues de 128 taureaux de testage et de 52 taureaux de service. Ceux-ci sont introduits dans l'analyse pour amliorer les connexions entre troupeaux. Le modle comporte l'e,!'et alatoire "pre de testage"et les effets fixs "pre de service", "troupeau-anne", "ge au premier vlage", "anne-saison de vdage" et "dure de la lactation prcdente". L'apparentement des reproducteurs mles est considr. Les donnes transformes par la dcomposition de Cholesky sont analyses par le maximum de vraisemblance restreint avec un algorithme apparent l'E.M. - Les corrlations gntiques des 6 caractres, toujours suprieures 0,89, sont lgrement plus faibles pour les lactations1 et 3. Pour les caractres de production, lhritabilit varie de 0,17 0,27. Les corrlations phnotypiques sont suprieures 0,60 pour les lactations successives et infrieures 0,55 pour les lactations1 et 3. Les taux prsentent une hritabilit suprieure 0,44 et des corrlations phnotypiques voisines de 0,7 et pratiquement indpendantes du couple de lactations considr. Ces rsultats indiquent que les diffrentes lactations peuvent tre traites comme des rptitions dun mme caractre. Ce modle, dit de &dquo;rptabilit&dquo; permet dallger les calculs sans diminuer lefficacit de la slection. bovins laitiers - production laitire - composition du lait - paramtres gntiques maximum de vraisemblance The goal of dairy selection is to improve lifetime production of cows, which implies taking into account the different lactations. Until now, genetic evaluation of the animals has in most cases been made under the assumption that these lactations are influenced by the same genes. In some countries only the first lactations are considered; in others the so-called &dquo;repeatability model&dquo; (Henderson, 1987) in which all lactations are treated as repetitions of one trait is fitted. But the lactations are made at various ages and physiological status of the animals and may therefore be determined somewhat by different genes. The accuracy of the genetic evaluation and thus the effi!ciency of dairy selection might be improved by fitting a multitrait model to the lactations. Reliable estimates of the genetic parameters for the different lactations are needed to appreciate this possible gain in accuracy. Data usually available for such estimations are selected as breeders cull about one quarter of the animals by the end of each lactation. Their decision is mostly based on dairy performance. Useful methods of estimation of these parameters have been available only recently. Hendersons methods (1953) assume animals are measured for all lactations, thus leading to results biased by the selection. However, the maximum likelihood (ML) (Hartley and Rao, 1967) and restricted maximum likelihood (REML) (Patterson and Thompson, 1971) estimators can take into account this selection (Im et al., 1987), a necessary condition being that the selection process is based only on the observed data or on observed data and independant variables. REML was prefered to ML as it accounts for the loss of degrees of freedom in simultaneous estimation of the fixed effects. Moreover, theoretical studies have shown that the optimum statistical procedure maximising the genetic merit of selected animals consists of estimating variance and covariance components by REML and thereafter applying these estimates in the mixed model equations (Gianola et al., 1986). MATERIALS AND METHODS Records for the first 3 lactations of Montb6liarde cows whose first calving occurred between 1/09/1979 and 30/08/1982 were extracted from the National Milk Recording files. The conditions of editing are presented in Table 1. Records made after cows changed herds were disregarded (Meyer, 1984). They represented 1.5% of the records for second lactation and 1.3% of the records for third lactation. Cows were nested Two populations of females were considered. The first was made of daughters of test bulls. It was used to estimate sire components of variance and covariance. The second consisted of daughters of the most widely used proven sires. As these bulls had been selected, they were treated as fixed effects and were not considered for the estimation of sire components. The performances of their daughters were introduced in the analysis in order to improve the accuracy of the estimation through additional information, increased herd size and degree of connectedness between herds. A total of 180 bulls of which 128 were random test bulls was considered. To simplify computation, records were split into 2 data sets, as did Meyer (1984, 1985a) and Swalve and Van Vleck (1987). The first data set consisted of the daughters of sampling bulls born in 1975 and the second of the (...truncated)