The objectives of this experiment with Hereford (Hf) and Charolais (Ch) bulls offered grass silage-based diets were to determine the effects on performance, carcass traits and meat quality of the proportion of concentrate in the diet, and the inclusion of rapeseed meal (RSM) in the barley-based concentrate. The two concentrate proportions were 200 and 500 g kg-1 dry matter, fed without or with RSM. The Ch bulls tended to achieve higher gain, produced less fat, had a higher percentage of meat from high-priced joints and had a lower degree of marbling in their meat compared to the Hf bulls. Dry matter and energy intakes, growth performance and carcass conformation improved with increasing concentrate level. Intake parameters and conformation improved more with the Ch bulls than with the Hf bulls as a consequence of increased concentrate allowance. RSM had only limited effects on the performance, carcass traits or meat quality.