Fleece rot and body strike in Merino sheep. II. Phenotypic and genetic variation in liability to fleece rot following experimental induction

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Abstract
Phenotypic and genetic variation in the susceptibility of Merino sheep to fleece rot and body strike were examined following experimental induction of fleece rot by wetting. A total of 949 Merino ewe hoggets, from 15 Merino bloodlines comprising 2 Fine-wool, 2 Medium Non-Peppin, 10 Medium Peppin and I Strong-wool flock were measured over a three-year period. Following wetting, 82.8'Yo of the hoggets developed fleece rot and 7.9% were affected with body strike. The incidence and severity of fleece rot was highest on the back, whereas body strike occurred mainly on the neck. Body strike was largely confined to the period of a flywave in the second year of the study. The low incidence of body strike reduced the reliability with which genetic variation in this trait could be estimated. Between-flock genetic variation in liability to induced fleece rot was largely strain variation (with increasing liability from Fine-wool to Medium-wool to Strong-wool strains). Variation between bloodlines within strains was not as large as those reported for the same bloodlines under natural conditions. The heritability ( ¦ s.e.) of liability to fleece rot was 0.86 using quasi-maximum likelihood procedures, which agrees well with the heritability of fleece rot incidence (0.40 ¦ 0.13) after transformation on to an underlying scale (0.87 ¦ 0.29) obtained through restricted maximum likelihood (KEML). Estimates of genetic parameters obtained for these data through REML procedures were in good agreement with estimates obtained through ordinary least squares analyses. Fleece rot induction allowed screening of a large number of sheep in a relatively short period for their susceptibility to fleece rot. Further evaluation is needed to determine the role of fleece rot induction in breeding programmes aimed at reducing the susceptibility of Merino sheep to natural fleece rot and body strike.