Disease resistance
The old-time breeders used to say to never use a
bird in the breeding pen that had been medicated that
year. While the bird may seem healthy, that the bird
suffered disease is one indicator of low immune function.
Also, in some cases of disease, the symptoms may
have dissipated but the animal may not have completely
recovered.
Culling all birds that become sick is one way to
positively select for disease resistance in breeding stock
within the region in which the flock is located.
Many poultry breeders have found that after a few generations
of culling all sick birds, illness will no longer be found
in the flock. This practice should not be expected to
work for highly pathogenic diseases.
Master breeder of Brown Leghorns, James P. Rines,
Jr., said many times, Your flock will have only what
you tolerate. This saying can be broadly applied to all
aspects of breeding, including disease resistance.