Thirty‑two early weaned Merino lambs were used to study the effect of roughage supplementation (cereal straw plus concentrate vs all‑concentrate diet) and two concentrate types (C: 75% barley, 20% soyabean meal, 2% molasses and 3% vitamins and minerals; M: 75% maize, 20% soyabean meal, 2% molasses and 3% vitamins and minerals) on feed intake, growth rate and carcass characteristics (carcass weight, morphology, pH and chilling losses, commercial yield and fat and meat color). There were neither significant interactions (P >  0.05) nor any significant difference due to roughage supplementation. Lambs fed maize‑based concentrate showed significantly (P > 0,05) higher values of carcass weight (12.4 vs 12.6 kg), commercial yield (46.2 vs 47.6%) and the ratio of carcass weight to carcass external length (252 vs 262 g/cm) than those receiving the barley‑based concentrate. These effects were probably due to a higher carcass fattening caused by differences in the protein/energy ratio between concentrate feeds. According to these results, roughage supplementation would not be necessary when barley or maize are given as whole grain. Nevertheless, it must be taken into account that lambs select concentrate ingredients and, in group feeding, there might be important individual differences in the composition of the selected diet and, therefore, in nutrient supply, growth rate and carcass quality.