Four, ruminal stulated swamp buffaloes with initial body weight (BW) of 420±15.0 kg, were randomly assigned to four dietary treatments according to a 2×2 factorial arrangement in a 4×4 Latin square design. The dietary treatments were as follow: untreated rice straw (RS) without EuO (T1) and with EuO (T2) supplementation, and 3% urea-treated rice straw (UTRS) without EuO (T3) and with EuO supplementation (T4). The EuO was supplemented at 2 mL/hd/d and both RS and UTRS were fed ad libitum. Concentrate (14.2% CP) was fed daily at 0.3% BW twice daily at 07.00 h and 16.00 h. All experimental animals were kept in individual pens with availability of clean fresh water and mineral blocks at all times. The experiment was conducted for 4 periods, and each period lasted for 21 d. During the first 14 d, DMI was recorded, while during the last 7 d, all animal were moved to metabolism crates for total feces and urine collections.