EFFECT OF CLENBUTEROL ON GROWTH, CARCASS AND SKELETAL-MUSCLE CHARACTERISTICS IN BROILER-CHICKENS

1. Male and female broiler chickens (144 in total) were given diets su pplemented with clenbuterol (CB) at 0 (control) and at 1 mg/kg between 28 and 49 d of age to study the effect of CB on growth, carcase and s keletal muscle. 2. CB improved growth in males by increasing daily wei ght gain and final live weight and by lowering food conversion ratio. In females it changed the carcase composition by reducing abdominal fa t pad and by increasing the proportion of protein. Consequently, carca se protein gain was increased in both sexes (11% and 16%, respectively ). 3. Skeletal muscle weights were enhanced by between 6% and 22%. Mus cle fibre diameters were increased in extensor hallucis longus (EHL) b ut no in gastrocnemius (GAS) muscle. This increase was more pronounced in females. EHL total muscle fibre number remained unchanged. The pro portion of fast-twitch glycolytic fibres was increased at the expense of fast-twitch oxidative fibres in males only. Nuclear/cytoplasm and D NA/protein ratios tended to be decreased by CB. 4. From the elevated E HL muscle RNA/DNA, unchanged protein/RNA and translation activity it i s suggested that CB stimulated protein synthesis at the pretranslation al level. Reduced protein degradation is deduced from decreased neutra l calcium-dependent proteolytic activity. 5. It is concluded that broi ler chickens respond to long-term CB treatment as has been shown in va rious mammals. However, the sex-specific response in growth, carcase c omposition and skeletal muscle cellularity is more clearly apparent in broiler chickens.