Multivariate restricted maximum likelihood estimation of genetic parameters for production traits in three selected turkey strains

Multivariate restricted maximum likelihood estimation of genetic parameters for production traits in three selected turkey strains H C hapuis M T ixier-Boichard V D ucrocq Summary - Genetic parameters related to growth, carcass composition and egg production were estimated on three (two female and one male) commercial strains of turkey using the method of restricted maximum likelihood (R.EML). In order to account for the sexual dimorphism in turkeys, body weight (BW, measured at 12 and 16 weeks of age) was considered as a sex-limited trait. As many as seven traits were analyzed simultaneously in one strain. Egg numbers were normalized using a Box-Cox transformation. Three different genetic models were used. The first one was a linear mixed model with a direct genetic effect. Model 2 accounted in addition for a dam's environmental effect, while model 3 introduced a maternal genetic effect. The heritability estimates of BW were very high, especially for female traits (0.77 for female BW16 and 0.68 for male BW16 in strain B). Sexual dimorphism was less heritable (0.23, 0.16, and 0.14 for the 16 weeks body weight sex difference in the three strains considered). One of the female strains exhibited a strongly negative genetic correlation (-0.5) between female BW and egg number. The elevated values of the estimates probably originated from the method used, which accounted for the bias due to the sequential selection that had been carried out, and from the choice of the base population. Use of models 2 and 3 resulted in slightly lower heritability estimates than model 1, due to low maternal effects. The latter, however, offered a reasonable compromise between quality and computational cost of the evaluations. turkey / genetic parameter / restricted maximum likelihood - Rsum - Estimation par maximum de vraisemblance restreinte des paramtres gntiques de caractres de production dans trois souches de dinde. Les paramtres gntiques de caractres relatifs la croissance (poids corporels 1,! et 16 semaines), la teneur en gras (mesure ultrasonique) et la ponte ont t estims laide de la mthode du maximum de la vraisemblance restreinte (REML) dans trois souches de dindes slectionnes. Les caractres de poids ont t spars selon les sexes, afin de rendre compte du dimorphisme sexuel important dans lespce et jusqu sept caractres ont ainsi t analyss simultanment dans une des souches. Les donnes de ponte ont t normalises laide dune transformation de Bo!-Cox. Trois modles gntiques diffrents ont t utiliss. Le premier est un modle linaire mixte incluant la valeur gntique additive individuelle comme effet alatoire. Dans les autres on ajoute un effet maternel dabord considr comme un effet essentiellement de milieu (modle 2) puis uniquemement gntique (modle 3). Les hritabilits sont trs fortes pour les poids corporels, plus leves pour les poids femelles que pour les poids mles (0,77 pour les femelles 16 semaines dans la ligne B contre 0,68 pour les mles). Le dimorphisme sexuel est un caractre plus faiblement hritable (0,23; 0,16; et 0,14 pour la diffrence de poids entre mles et femelles 16 semaines dans les trois lignes). Dans une des lignes femelles, la corrlation gntique est fortement ngative (-0,5) entre le poids des femelles et le nombre dceufs pondus. Les valeurs leves des paramtres gntiques sexpliquent probablement par la mthode employe qui permet de prendre en compte le biais important li la slection de type squentiel. Le choix de la population de base permet galement de!pliquer ces valeurs inhabituelles. Les modles 2 et 3 donnent des estimes lgrement moins leves pour les hritabilits que le modle 1, cause de la faiblesse des efJ&dquo;ets maternels. Le modle 1 permet nanmoins un bon compromis entre simplicit des calculs et qualit de la description. dinde / paramtre gntique / maximum de vraisemblance restreinte Poultry breeding is characterized by large populations subject to few environmental effects (often accounted for in evaluations as a unique contemporary group, ie, hatch effect). This explains why selection index theory has been used successfully for the past few decades, while analysis of (co)variances (ANOVA) type methods were used to estimate genetic and phenotypic correlations. Despite its simplicity and its properties, selection index theory is open to improvement, most notably because it does not account for possible differences in expected values between contemporary groups and/or generations, or for changes in additive genetic variances due to selection, inbreeding, and preferential matings (Bulmer, 1971). As a result, since Hendersons pioneering work (1973), the methodology of best linear unbiased prediction applied to an animal model (BLUP-AM) has been developed in many livestock species for routine genetic evaluations. This method requires knowledge of variance components in a supposedly unselected and unrelated base population. Yet genetic parameters have to be estimated from available data. Despite the computational difficulty, the method of restricted maximum likelihood (REML) presented by Patterson and Thompson (1971) has been shown to have most desirable properties, mainly because of its ability to correct for bias due to selection (Gianola et al, 1986) . Poultry breeding companies have only lately come to use these more advanced evaluation methods, certainly because the need to use them seemed less stringent than for other livestock species (Hartmann, 1992). For example, Besbes et al (1992, 1993) recently illustrated their use in selection of laying hens. Breeding of meat-type poultry is done under quite different circumstances from those of laying hens, because of the peculiar selection scheme where birds are sequentially measured, evaluated and culled. The bias involved in the last evaluation stages may be considerable when the selection based on the previous step is not accounted for. In such a situation, it is preferable, although often computationally demanding (Ducrocq, 1994), to use a multitrait approach and include all records on which selection is based. Better use of the available information results in greater accuracy and reduces systematic biases in estimates of population genetic parameters and BVs. For example, it may be beneficial to undertake a joint estimation of genetic parameters for reproductive and growth traits in turkeys because 1) reproductive traits are measured on a restricted fraction of the population; 2) there are missing records for some traits, which is the outcome of selection based on body weight; and 3) intense selection on both growth and reproductive traits has been carried out for many generations. This study aims to estimate genetic parameters of production traits in selected turkey strains using REML methodology with an animal model. MATERIALS AND METHODS Data and description of traits This study was based on data from three selected strains of turkeys, referred to as strains A, B and C. Strains A and B are female line (...truncated)