In a number of studies with beef cattle, diets composed mainly or entirely of grass silage have sustained lower growth rates and have resulted in a greater proportion of fat in the gain than high-concentrate diets offered at the same level of energy intake. However these have involved replacement of silage by concentrates in the diet and there would appear to be no comparative information on responses to adding silage or concentrates to a mixed diet. It is also not clear to what extent the lower growth rates sustained by silage are due to the higher energy content of the gain in animals given silage or to lower efficiency of utilization of metabolisable energy. A comparative slaughter experiment has been carried out to examine responses to the addition of silage or concentrates to the diet of lambs.