Association of flystrike with production, visual and health traits in Australian Merino sheep

Context Flystrike is a major welfare and productivity challenge in Australian Merino sheep, and its complex biological basis makes genetic improvement an essential long-term control strategy. However, the relationships between flystrike incidence and routinely recorded production, visual, and health traits are poorly understood. Aims This study examined genetic and phenotypic correlations between flystrike incidence (early and late breech and body strike) and a comprehensive set of production, visual, and health traits in Merino sheep. Methods Flystrike phenotypes were sourced from historic research and development flocks and ram breeder flocks participating in the Flystrike Genomics Reference Flock Project. Production, visual, and health traits records were obtained from MERINOSELECT. Genetic parameters were estimated using linear mixed models. Key results Results revealed a complex genetic interplay among productivity and flystrike resistance traits. Flystrike susceptibility was heritable, indicating potential for genetic improvement through selection. Phenotypic correlations between flystrike and the other traits were generally low, whereas moderate to high genetic correlations were observed with several fleece and visual traits. Fleece weights had unfavourable positive genetic correlations with flystrike, particularly body strike, indicating that selection for higher fleece production may inadvertently increase susceptibility. In contrast, body weights had low to moderate favourable negative correlations with flystrike, indicating that heavier sheep were associated with lower flystrike incidence. Fibre diameter had a negative correlation with breech strike, suggesting finer-wool sheep are more prone to being struck, while higher fibre diameter variability and shorter staple length were associated with increased flystrike risk. Among visual traits, breech wrinkle, dag, and breech cover had strong favourable genetic correlations with breech strike. Fleece rot and wool colour were more strongly associated with body strike, supporting their potential use as indirect indicator traits for this type of flystrike. Fleece rot and dag were key predisposing factors for flystrike, especially at early ages. Conclusion These findings demonstrate that genetic improvement for flystrike resistance is achievable but must be balanced against productivity goals. Implications Integrating direct flystrike phenotypes with key visual indicator traits into multi-trait evaluation systems will enhance breeding value accuracy and support sustainable genetic gains for flystrike resistance within Merino breeding programs.