Breeding for improved disease resistance in organic farming--possibilities and constraints.

Lowered incidences of disease may be reached in several ways: management and rearing measures, vaccination programmes and preventive medications as well as breeding for improved disease resistance. Here the focus is on breeding for improved resistance to infectious diseases. In comparison to conventional farming, one has to acknowledge that the spectrum of diseases in animals reared under organic conditions is different and that the proportion of the breeding stock of animals in organic farming is considerably smaller. There are at least four different approaches that may be used in breeding towards resistance to infectious diseases. The most obvious is to record disease incidence in the progeny and select those parents that produce the progeny with the lowest incidences of disease. Another approach is to use breeders possessing certain major histocompatibility complex antigens suggested being associated with resistance to certain infections. A third approach is to analyse the heritability of a set of immune functions or related traits crucial for resistance to infections and then use the traits with high heritability in breeding programmes. Finally, one may genetically select animals for high immune response using an index that combines estimated breeding values for several immunological traits. Examples of these various approaches are given and the feasibility for using these in organic farming are discussed.