Threshold model analysis of lamb survivability in Romney sheep

Abstract

Lamb survivability is a major determinant of the reproductive efficiency of the flock. The objective of this study was to estimate variance components for direct, maternal, and permanent environmental components of lamb survivability. Threshold model procedures were used to estimate variance components from 25 362 lamb survival observations representing 136 sires from two Romney flocks in New Zealand with an average lamb survival of 80.8%. Lambs were born between 1997 and2000. Fixed effects fitted in the model included sex and contemporary group, defined as the interaction of age of dam, flock, and birth date class. Median heritabili-ties were 0.106 ± 0.010 for direct, 0.082 ± 0.005 for maternal, and a median proportion of phenotypic variation 0.098 ± 0.003 for permanent environmental effects. The direct-maternal correlation was –0.75 ± 0.03. Estimated breeding values on the underlying scale were back-transformed to the phenotypic probability scale. For direct lamb survivability, the highest performing sire's lambs had a 14.8% greater chance of survival than the lowest performing sire's lambs for flocks with an average lamb survivability of 80%. Although heritabilities for both components of lamb survivability are low, selection for this trait can be accomplished and is of economic importance for the sheep breeder. © 2006 Taylor & Francis Group, LLC.