Abstract:
The conceptual model that variation in resistance of Merino sheep to fleece rot and body strike could be attributed to an underlying variable with a continuous normal distribution, termed 'liability', was examined in four experiments. A total of 640 young Merino sheep were exposed from 0 to 13 days to a simulated rainfall treatment to allow the development of fleece rot. The incidence of fleece rot ranged from 0% of sheep affected in unwetted control groups and increased up to 100% affected with increased exposure to wetting. Assuming binomial variation, the response in fleece rot and flystrike incidence was fitted on an underlying scale using generalized linear models. In the case of fleece rot severity, grades of exudative fleece rot were shown to fit an ordered categorical response model. The assessment, induction and liability of sheep to fleece rot were found to be highly repeatable in all three cases, indicating that liability to fleece rot could be accurately screened for using experimental induction. Fleece rot severity was found to be an accurate indicator of liability to body strike but not of other forms of flystrike. The possible mechanisms and factors contributing to the liability of sheep to fleece rot and body strike have been discussed, and moisture status of the fleece during the development of the two diseases was indicated as an essential common denominator.