The concentration of total VFA was not changed by the feed block and the mean values ranged from 116.8 to 119.6 mmol/L.Foiklang et al. (2011) reported that total VFA concentrations in the rumen of buffalo fed with the feed block ranged from102.2 to 116.0 mmol/L. Supplementation of U-cas in feed block enhanced the proportion of propionic acid and deceasedacetic acid concentration. Similarly, Cherdthong et al. (2011a) reported that propionic acid concentrations were increasedby U-cas supplementation in concentrate diet of dairy cows. This could improve energy use for ruminant because propionicacid maintains a positive relationship with energy utilization efficiency. Thus, it means that higher propionic acid indicateda better energy yield while shifting in acetic: propionic acid and acetic plus butyric acid: propionic acid ratio explainedthe better efficiency of energy use in cattle consumed feed block containing 180 g/kg of U-cas. These findings agreed withCherdthong et al. (2011a,b) who found a higher propionic acid and lower acetic: propionic acid ratio in the ruminal fluid ofcows fed a high grain diet.