Antibody titers against IB on 0, 14 and 28 days post-inoculation. * Antibody titers at day 0 were lower in the challenge group than in the control group for Ju-Chi and SPF. The effects of breed and H6N1 challenge, without any interaction, were still observed at day 14 post-inoculation. Higher antibody levels were found in the control group for all breeds. Nagoya was the only one to exhibit a stronger response to IB vaccine in the H6N1 control group at day 14.