Twelve British Friesian and 12 Ayrshire castrated male calves were fed ad libitum one of three complete diets of differing energy concentration from approximately 140 to 410 kg live weight. Differences in energy concentration of the diets were achieved by inclusion of different proportions of wet distiller's grains (WDG). Increasing the proportion of WDG in the diet caused reductions in performance. This effect was more marked with the Friesians than with the Ayrshires. The treatments had small and generally non-significant effects on carcass conformation and composition.