This study was conducted to investigate the effect of different levels of pomegranate marc (PM) supplemented with or without polyethylene glycol (PEG) on performance, intake, nutrient digestibility, nitrogen retention, ruminal ammonia concentration and protozoal population in growing lambs. Thirty Mehraban male lambs (4–5 months of age; 29.5 ± 1.19 kg initial body weight (BW)) were assigned randomly to five treatments (6 lambs per treatment) which were different levels of PM with or without PEG: without PM (Control), 80 g PM/kg DM (PM80), PM80 with PEG (PM80 + PEG), 160 g PM/kg DM (PM160), and PM160 with PEG (PM160 + PEG). On day 61 of the fattening period, four animals from each group were selected randomly and housed individually in metabolism crates to evaluate nutrient digestibility by total collection method. Control animals had a greater dry matter intake (DMI) and average daily gain (ADG) compared with PM160 (P < 0.05). However, lambs fed the high level of PM showed decreased DMI and ADG compared with those fed the low level of PM (P < 0.05). The addition of PEG to the PM diets increased DMI (P < 0.05), but did not affect the ADG (P > 0.05) of lambs. The PM160 diet had lower crude protein (CP), neutral detergent fiber (NDF) and acid detergent fiber (ADF) digestibities compared with other treatments (P > 0.05). The addition of PEG to the PM diets increased the digestibilities of OM, CP, NDF and ADF (P < 0.05) by lambs. There were no differences among treatments for N intake, fecal N output and N retention (P > 0.05). Addition of PEG to the PM diets increased urinary N output and the ratio of urinary to fecal N (P < 0.05). Ruminal ammonia concentration, numbers of Entodinium spp. and total protozoa population were higher in control-fed lambs than lambs fed other treatments. Inclusion of PEG in diets containing PM increased ruminal ammonia concentration, blood urea nitrogen, numbers of Entodinium spp. and total protozoa population (p < 0.05). In conclusion, addi
