The experiment was carried out using a 4 × 4 factorial design. There were 16 experimental groups. Each group consisted of 3 lambs. The experiment lasted 56 days. During the experiment, the oregano oil (0 and 5 g/kg DM) and urea (0, 6, 12, and 18 g/head per day) were given to lambs consuming low energy/low protein diets (2350 Kcal/kg DM/ 10.80% CP/DM, respectively) and normal energy/normal protein (2600 Kcal/kg DM, 14.97% CP/DM, respectively) diets. Daily body weight gain of lambs consuming normal energy/normal protein diets and supplemented with 6, 12 and 18 g/head per day urea were significantly (P < 0.01) higher compared to those consumed low energy/low protein diets and supplemented with oregano oil. The voluntary feed intake of lambs consuming the normal energy/normal protein diets supplemented with urea (6, 12 and 18 g/head per day) was lower compared to lambs consuming low energy/low protein diets supplemented with oregano oil and urea. On the other hand, the feed efficiency of lambs consuming the normal energy/normal protein diets supplemented with urea (6, 12 and 18 g/head per day) was significantly (P < 0.01) higher compared to lambs consuming the low energy/low protein diets supplemented with oregano oil and urea. The supplementation of urea to lambs consuming the low energy/low protein diets and the normal energy/normal protein diets increased the weight of cold carcass. The supplementation of urea had no significant (P > 0.01) effect on the other carcass characteristics.

The experiment was carried out using a 4 × 4 factorial design. There were 16 experimental groups. Each group consisted of 3 lambs. The experiment lasted 56 days. During the experiment, the oregano oil (0 and 5 g/kg DM) and urea (0, 6, 12, and 18 g/head per day) were given to lambs consuming low energy/low protein diets (2350 Kcal/kg DM/ 10.80% CP/DM, respectively) and normal energy/normal protein (2600 Kcal/kg DM, 14.97% CP/DM, respectively) diets. Daily body weight gain of lambs consuming normal energy/normal protein diets and supplemented with 6, 12 and 18 g/head per day urea were significantly (P < 0.01) higher compared to those consumed low energy/low protein diets and supplemented with oregano oil. The voluntary feed intake of lambs consuming the normal energy/normal protein diets supplemented with urea (6, 12 and 18 g/head per day) was lower compared to lambs consuming low energy/low protein diets supplemented with oregano oil and urea. On the other hand, the feed efficiency of lambs consuming the normal energy/normal protein diets supplemented with urea (6, 12 and 18 g/head per day) was significantly (P < 0.01) higher compared to lambs consuming the low energy/low protein diets supplemented with oregano oil and urea. The supplementation of urea to lambs consuming the low energy/low protein diets and the normal energy/normal protein diets increased the weight of cold carcass. The supplementation of urea had no significant (P > 0.01) effect on the other carcass characteristics.