Understanding parasitic infection in sheep to design more efficient animal selection strategies.

Modern livestock breeding practices provide new opportunities for producing animals that are adapted to their production environment and are free of disease. Using current knowledge of biology and by seeking 'the desired outcome' animal selection strategies can be designed that deliver more precisely defined results so maximising genetic gain and minimising risk. This review briefly describes the evolution of genetic selection in livestock and considers some of the positive and negative aspects of selection practices over time. The selection of sheep to withstand gastro-intestinal nematode parasitism is used as an example to explain how developments in selection strategy have improved genetic progress for complex traits. Re-evaluation of the understanding of the outcomes of selection for parasite resistance is used here to examine whether a more sophisticated approach is desirable, and to propose a number of additional phenotype measurement strategies that could complement and improve the quality of information used for animal selection. Finally some ideas are presented for creating a situation where a designed, highly defined breeding objective might be used to increase precision and reduce risk. This may become possible via research to adapt or develop tools for more sophisticated phenotypic evaluation, to discover biological processes integral to desired breed changes, and to define desired animal types which match economic and societal expectations.