Forty growing ram lambs (Lori-Bakhtiari breed) averaging 28 kg (±1.5) were allocated in this study to evaluate the optimal dietary energy density in a lamb fattening enterprise in Iran. The feed intake (FI), body weight (BW), gain to feed ratio (G:F) and carcass characteristics were studied. Four treatments differed in metabolizable energy content (ME) as follow; 2, 2.3, 2.6 and 2.9 Mcal/kg for treatments 1 to 4, respec-tively. The study lasted 120 days and all the experimental lambs were slaughtered and carcass composition was compared among treatments. Average daily gains were 230, 252, 267 and 259 g/d for treatments 1 to 4, respectively (P<0.01). Moreover, lower energy density resulted in up to 19% greater FI (P<0.01) and higher F:G ratio (P<0.01) and therefore lower carcass growth efficiency (P<0.05). Higher energy density improved G:F ratio; but caused fatter carcasses compared to lower dietary energy levels. In addition to fatter carcasses for higher energy diets, the greatest tail-fat weight was observed for treatment 4 (2.9 Mcal/kg) and may explain the lower feed efficiency in this treatment. The values for lean mass weights were 12.7, 14.1, 14.7 and 12.7 kg for treatments 1 to 4, respectively. Overall, the best carcass efficiency was found with treatment 3. Based on these results, energy density of 2.6 Mcal/kg may be recommended for growing Lori-Bakhtiari male lambs in a fattening enterprise under the conditions of this trial.
