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Title: THRESHOLD MODEL ANALYSIS OF LAMB SURVIVABILITY IN ROMNEY SHEEP

Author
item Wilson, Carrie - Welsh
item GARRICK, DORIAN - COLORADO STATE UNIVERSITY
item ENNS, RICHARD - COLORADO STATE UNIVERSITY
item NICOLL, GEOFF - LANDCORP FARMING LIMITED

Submitted to: New Zealand Journal of Agricultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2006
Publication Date: 12/1/2006
Citation: Welsh, C.S., Garrick, D.J., Enns, R.M., Nicoll, G.B. 2006. Threshold model analysis of lamb survivability in romney sheep. New Zealand Journal of Agricultural Research. 49:411-418.

Interpretive Summary: 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. These estimates can be incorporated into a genetic evaluation to select animals that are superior for lamb survival. Threshold model procedures were used to estimate variance components from 25,362 lamb survival observations with 136 sires from two Romney flocks in New Zealand with an average lamb survival of 80.8%. Fixed effects fitted in the model included sex and contemporary group, defined as the interaction of age of dam, flock, and birth date class. Heritabilities were 0.106 plus or minus 0.010 for direct, 0.082 plus or minus 0.005 for maternal, and 0.098 plus or minus 0.003 for permanent environmental effects. The direct-maternal correlation was -0.75 plus or minus 0.03, indicating selection for one component of the trait is expected to decrease performance for the other component of the trait. 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.

Technical 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 with 136 sires from two Romney flocks in New Zealand with an average lamb survival of 80.8%. 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 heritabilities were 0.106 plus or minus 0.010 for direct, 0.082 plus or minus 0.005 for maternal, and 0.098 plus or minus 0.003 for permanent environmental effects. The direct-maternal correlation was -0.75 plus or minus 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.