Abstract:
The prevalence of body strike was monitored in 740 mixed sex weaner progeny from 43 sires, representing 15 different Merino genotypes. A flywave, characterized by a sudden onset in the prevalence of flystrike, occurred over a 2 month period which coincided with unseasonally high rainfall. A total of 190 strikes were treated in 176 sheep representing a strike rate of 25.7 strikes per 100 sheep and a prevalence of 23.8%. The majority of strikes (92.6%) were body strikes with 19.7% of all sheep affected. The heritability of susceptibility to body strike expressed as prevalence was 0.26�_0.12 for both least squares (LS) and restricted maximum likelihood (REML) procedures. The heritability of liability to body strike using quasi-maximum likelihood procedures and assuming a continuous underlying scale was 0.53 which agreed well with estimates from the LS and REML analysis after transformation (0.54�_0. 25). Predicted genetic progress in the reduction of susceptibility to body strike was calculated for mass selection where the best 5% of rams and 60% of ewes were selected, with a starting prevalence of 20% body strike, assuming (a) an environment exposed to continuous flywaves, and (b) an environment with infrequent flywaves. After 20 years of direct selection the prevalence of body strike would be reduced to 7.2% and 16.1% for the two environments respectively. Indirect selection based on (c) a variable prevalence of fleece rot with a long term average of 30% and (d) fleece rot expressed at 80% would reduce the prevalence of body strike to 6% and less than 2% respectively. Indirect selection based on (e) a hypothetical indicator trait with a continous distribution, a high heritability (0.4) and a moderate genetic correlation with liability to body strike (0.4), would also reduce the prevalence of body strike to less than 2% after 20 years of selection.