A comparative slaughter experiment was carried out to examine the effects of feeding either grass silage alone or a mixed diet of silage and concentrate on animal performance and the partitioning of nutrients between lean and fat deposition in steers. Eighteen Hereford×Friesian steers were randomly assigned to two dietary treatments; grass silage only (S) or a mixture of grass silage and a barley/soya bean meal concentrate (80[ratio ]20 on fresh basis) in the ratio of 60[ratio ]40 (on a metabolizable energy basis; SC), and to one of three target slaughter liveweights, 250, 350 or 500 kg. Metabolizable energy (ME) intake was maintained at 800 kJ ME per kg metabolic liveweight (kg0·75) per day. The relationships between chemical composition and empty body weight (EBW) at slaughter were assessed using allometric equations (logey=logea+blogeEBW). When assessed across the slaughter weights, supplementing silage with concentrates resulted in higher rates of liveweight (P<0·001), carcass fat (P<0·05) and protein (P<0·01) gains and a reduction in time taken to reach the average slaughter weight by 57 days (P<0·001). Carcass protein deposition was relatively linear across the slaughter weights (250–500 kg) and the relationship with EBW was Y=0·2372X0·8831 across treatments and did not differ between the diets. The rate at which carcass fat was deposited in relation to weight across all steers was Y=0·0004X1·9648 and was not different between the two diets. Hence, ratios of carcass fat[ratio ]protein and carcass fat[ratio ]protein gain ratios were not different. The results suggest that the main effect of feeding grass silage compared with grass silage-concentrate at similar levels of ME intake was to increase the rate of tissue accretion, but nutrient partitioning between fat and protein deposition was unchanged. There was no evidence of increased carcass fat[ratio ]protein deposition in silage-fed animals, which suggests that there is no problem of greater fat and reduced protein deposition in animals fed higher quality grass silage.