AbstractThis study was aimed to first, determine the nutritional value ofBerberis vulgarisleaf (BVL), using in vitro gas production technique and second, determine the effect of replacing alfalfa hay (AH) with BVL in lamb diets on nutrient intake, performance, and carcass traits. In vitro rumen gas kinetics and fermentation profile were assessed using three fistulated lambs and 96 h incubation of samples. For the in vivo trial, 21 Baluchi male lambs of 5–6 mo of age and 30.6 ± 1.28 kg body weight (BW) were randomly assigned to three treatment diets containing BVL at 0% (CTRL), 7.5% (BVL7.5), and 15% (BVL15) of the total dry matter (DM) inclusion. The study lasted 84 d, which included 14 d for adaption and 70 d for sample collection. In vitro results showed that BVL had lower gas yield (GY24,P≤ 0.05) than AH. In vivo trial revealed that DM intake increased with BVL15 followed by BVL7.5 (P≤ 0.05). Digestibility of DM, organic matter, NDF, and acid detergent lignin decreased (P≤ 0.05) with BVL15. Total weight gain, average daily gain, hot carcass weight, and cold carcass weight were decreased (P≤ 0.05) with BVL15, however, no differences were observed between the BVL7.5 and CTRL groups. Feed cost per kilogram BW gain decrease (P≤ 0.05) by increasing the inclusion levels of BVL. In conclusion, BVL (up to 7.5% DM of diet) have positive consequences on performance, nutrient intake, rumen fermentation without deleterious effects on fattening performance.