Forage based feeding systems are often disadvantaged compared with those based on high cereal usage in terms of feed intake, live weight gain and efficiency of utilisation of dietary energy and protein. Furthermore, under some situations, particularly with animals fed on grass silage, cattle often have higher fat:protein carcass ratios than those fed other forage-diets. However, other factors such as age, genotype and physiological state may also influence nutrient partitioning. An experiment was conducted to investigate the effect of diet (based on silage alone or supplemented with additional energy and/or protein) and stage of development on the partitioning of nutrients between fat and lean deposition. Overall effects of diet on animal performance and carcass composition were reported by Scollan et al. (1999).