Effect of early life selection using indirect characters on the subsequent incidence of fleece rot in a flock of South Australian Merino ewes

Abstract

Correlations were estimated between 24 fleece and body characters measured at 1, 2, 3 and 4 years of age, and fleece rot induced by artificial wetting at 4 years of age in South Australian Merino ewes. The character most strongly and consistently related to fleece rot was greasy wool colour score which was correlated with fleece rot score when measured at all 4 ages (r = 0.21, 0.22, 0.31 and 0.26 respectively) and with liability to fleece rot when measured in 3 and 4-year-olds (r = 0.25, 0.38). A number of discriminant functions were developed from characters measured at each age and used to predict fleece rot susceptibility at 4 years of age. Culling 45% of hoggets on the basis of a function comprised of all characters measured would have reduced fleece rot by 39.3%. Culling on the basis of functions comprised of more manageable groups of characters (i.e. selected as (i) contributing significantly to the function when all characters were included, or (ii) on the basis of previous recommendations) would have reduced fleece rot by between 7.4 and 15.7% when 45% of hoggets were culled. Lower culling rates gave less reduction in fleece rot, except where the reduction was very small regardless of the per cent culled. It is concluded that in most practical situations there will be limited benefit from selecting ewes on the basis of indirect characters to reduce fleece rot susceptibility in the current generation.