THE EFFECTS OF DIETARY ENERGY AND CRUDE PROTEIN-CONCENTRATION ON GROWTH AND SERUM INSULIN-LIKE GROWTH FACTOR-I LEVELS OF CATTLE THAT DIFFERIN MATURE BODY-SIZE

An experiment was conducted to examine the influence of dietary energy level (11.17 and 11.88 MJ ME kg-1 DM) and crude protein (CP) level (1 1, 13 and 15 % DM basis) on growth and carcass composition and plasma insulin-like growth factor-I (IGF-I) levels of medium frame (MF) and l arge frame (LF) cattle. Dietary energy level or CP level did not influ ence the growth of MF steers. Averaged across energy levels, the growt h of LF steers increased linearly (P < 0.01; R2 = 0.60) with dietary C P level. No influence of protein level was observed on carcass tissue gain. Apparent CP and DM digestibilities increased (P < 0.05) with inc reasing CP level on low- but not high-energy rations. Serum IGF-I leve ls were higher (P < 0.05) in MF steers and in animals fed high-energy rations. Little or no association was observed between serum IGF-I lev els and indices of growth or carcass composition, It was concluded tha t the growth of LF steers was improved on diets formulated to CP level s in excess of those currently recommended. However, the biological me chanism responsible for the improved growth is not clear.