Metabolic disturbances in male broilers of different strains. 2. Relationship between the thyroid and somatotropic axes with growth rate and mortality

Seven male broiler strains (Arbor Acres, Avian Farms, Cobb-500, Hubbard-Pet erson, ISA, Naked Neck, and Ross) were compared for their growth rate, feed efficiency, and mortality due to sudden death and ascites. In addition, we ekly plasma levels of thyroid hormones [3,3',5-triodothyronine (T-3) thyrox ine (T-4), T-3: T-4 ratio, growth hormone (GH), and insulin-like growth fac tor-I (IGF-I)] were determined. The highly productive, commercial strains w ere very similar in their endocrine profiles but differed markedly from the Naked Neck chickens. Naked Neck chickens were characterized by higher plas ma Tg and lower T-4 levels at similar ages as well as when compared on the same body weight basis. The present findings support the hypothesis that th e slightly hypothyroid state of high productive broilers renders them more sensitive to metabolic disorders. Naked Neck chickens also had higher plasm a GH levels than those of their age-matched commercial broilers. The coeffi cient of variation for GH was highest for Naked Neck chickens, which is ind icative for an amplified CH burst amplitude. It may be stated that changes in plasma thyroid hormone concentration in indirect response to selection f or low feed conversion and fast growth may be causatively linked to suscept ibility for metabolic disturbances such as sudden death syndrome and ascite s.