Genetic parameter estimates for lamb survival in Romney sheep

Lamb survival is a trait of economic importance in sheep production. Losses may be attributed to the lamb or to the behaviour of the dam. Maternal effects are environmental for the offspring but are at least partly genetic for the dam. The genetic evaluation of survival is complicated by the fact that this trait has a binomial distribution whereas most analytical procedures assume normality. The objective of this study was to estimate heritabilities and genetic correlations for direct and maternal genetic effects using two underlying scales (probit and logit) for lamb survival in order to implement a new procedure of genetic evaluation. Variance components were obtained by Restricted Maximum Likelihood (REML) procedures after transformation of the data using a linear mixed model. Records of survival were from 25,874 lambs born over the period 1989 to 1995 in a prolific Romney flock representing the progeny of 218 sires and 6,771 dams. The model included fixed effects of sex, year, litter size at birth and age of dam and the random effects for direct genetic, maternal genetic and maternal environmental influences. Lambing and survival-to-weaning percentages were 178% and 87%, respectively. The estimates of heritability on the logit scale were 0.01, 0.03 and 0.04 for direct, maternal and total genetic effects, respectively. The proportion of total variance explained by maternal environmental influences was 0.09. The estimated genetic correlation between direct and maternal genetic effects was n0.26. Similar estimates were obtained on the probit scale. Estimates will be lower for the observed scale. These results indicate a greater opportunity for improving survival by manipulating the environment rather than by selection. However survival analysis is an important component of quality assurance in a genetic improvement scheme even if the resultant breeding values are not used for direct selection.

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