Four, Thai native beef cattle with initial body weight (BW) of 100 ± 3.0 kg were randomly assigned according to a 4 × 4Latin square design to receive feed blocks containing different levels of U-cas supplementation at 0, 120, 150 and 180 g/kgDM. A concentrate mixture (Table 2) was fed to animals at 5 g/kg of BW daily and offered in two equal meals per day at 7:00and 16:00. Cattle were fed rice straw by allowing for refusals of 100 g/kg. All animals were kept in individual pens. Cleanfresh water and feed blocks were available at all times. Individual intakes of rice straw, concentrate and feed blocks wererecorded daily by weighing the offered and refused feeds during the morning feeding. The experiment was conducted for4 periods with 21 days per each. The first 14 days were for adaptation period and last 7 days were for samples collectionas animals were moved to metabolism crates and fed the straw at 900 g/kg of the previous voluntary feed intake of straw.Concentrate was still offered at 5 g/kg of BW daily and feed blocks were available at all times for the whole experiment.