The effects of dietary energy and crude protein concentration on growth and serum insulin-like growth factor-I levels of cattle that differ in 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 (11, 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 large frame (LF) cattle. Dietary energy level or CP level did not influence the growth of MF steers. Averaged across energy levels, the growth of LF steers increased linearly (P < 0.01; R 2  = 0.60) with dietary CP level. No influence of protein level was observed on carcass tissue gain. Apparent CP and DM digestibilities increased (P < 0.05) with increasing CP level on low- but not high-energy rations. Serum IGF-I levels 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 levels and indices of growth or carcass composition. It was concluded that the growth of LF steers was improved on diets formulated to CP levels in excess of those currently recommended. However, the biological mechanism responsible for the improved growth is not clear. Key words: Steers, frame size, protein, energy, IGF-I