Genetic parameters for litter size in sheep: natural versus hormone-induced oestrusSteven Janssensa, Walter Vandepittea and Loys Bodinb
a K.U. Leuven, Centre for Animal Genetics and Selection, Department Animal Production, Kasteelpark Arenberg 30, 3001 Leuven, Belgium
b Station d'amélioration génétique des animaux, Institut national de recherche agronomique, BP 27, 31326 Castanet-Tolosan, France
(Received 7 January 2004; accepted 27 April 2004 )
Abstract - The litter size in Suffolk and Texel-sheep was analysed using REML and Bayesian methods. Litters born after hormonal induced oestrus and after natural oestrus were treated as different traits in order to estimate the genetic correlation between the traits. Explanatory variables were the age of the ewe at lambing, period of lambing, a year*flock-effect, a permanent environmental effect associated with the ewe, and the additive genetic effect. The heritability estimates for litter size ranged from 0.06 to 0.13 using REML in bi-variate linear models. Transformation of the estimates to the underlying scale resulted in heritability estimates from 0.12 to 0.17. Posterior means of the heritability of litter size in the Bayesian approach with bi-variate threshold models varied from 0.05 to 0.18. REML estimates of the genetic correlations between the two types of litter size ranged from 0.57 to 0.64 in the Suffolk and from 0.75 to 0.81 in the Texel. The posterior means of the genetic correlation (Bayesian analysis) were 0.40 and 0.44 for the Suffolk and 0.56 and 0.75 for the Texel in the sire and animal model respectively. A bivariate threshold model seems appropriate for the genetic evaluation of prolificacy in the breeds concerned.
Key words: sheep / litter size / oestrus induction / heritability / genetic correlation
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© INRA, EDP Sciences 2004