The present was aimed to study the effect of varying levels of niacin supplementation (0, 200, 400 and 600ppm, respectively) on medium urea based total mixed ration (TMR) replacing 20% of total crude protein (CP) of ration with different non-protein nitrogen (NPN) sources by in vitro gas production technique. All the rations were iso-nitrogeneous in nature. On the basis of higher partition factor, TD%, OMD%, microbial mass production and efficiency of microbial mass production. Supplementation of niacin in low urea based TMR did not have any significant effect on microbial mass production and its efficiency. The in vitro pH and NH3 concentration was signifi cantly reduced (p<0.05) at 600 ppm level of niacin supplementation. The TVFA concentration was signifi cantly comparable in control and urea based TMR and lowest in uromol based TMR. Niacin supplementation produced significantly higher (p<0.05) TVFA at 400 ppm level and lowest at 600 ppm in TMR. It can be concluded that slow release urea seems to be better option than urea and uromol as NPN supplement in the diets of ruminants

when medium (20% of total CP) urea based TMR is to be prepared.