Breeding for robustness in cattle

Breeding for enhanced resistance to infectious disease is an effective means of improving the health, fitness and robustness of ruminant livestock. The most amenable endemic diseases to genetic selection are likely to be mastitis, bovine leukaemia, gastrointestinal (GI) parasitism, tuberculosis (TB) and paratuberculosis in cattle; and mastitis, GI parasitism and footrot in sheep. For bovine mastitis, selection on clinical signs or somatic cell count (SCC) is well established; however the longterm wisdom of decreasing SCC is often questioned. A solution may be to decompose SCC into baseline and response variables, along with a liability to become infected, and select for reduced liability. Genetic markers are often sought for mastitis resistance, but the complexity of the host-pathogen interactions may mean that individual markers are of insufficient value, requiring whole-genome approaches. Bovine TB is an emerging zoonotic threat, and a disease of major importance in some geographical regions. Studies are currently underway to assess options for breeding cattle for increased resistance. In particular, identification of cases and controls from herds with disease outbreaks may allow efficient genome scans. Paratuberculosis is a similar bacterial disease, but its prevalence is currently unknown and improved diagnostic tests are required before effective genetic approaches can be contemplated. For nematode resistance and footrot in sheep, readily measured indicators of relative resistance are available, and observed genetic gains may be larger than predicted by genetic theory, due to decreased contamination from selected animals. Although it has yet to be explored, this result is also likely to apply for several dairy cattle diseases. In summary, selection for increased resistance to specific diseases may have considerable benefits for cow health, welfare and robustness. However, this conclusion is dependent upon the disease prevalence, the higher the prevalence the greater the benefits of selection and the greater the impact on robustness.