. The aim of this research was to find the optimum level of substitution of fresh Napier Grass with Spent Rice Straw Compost (SRSC) on rumen metabolism products. Two male cattle breeds consisted of indigenous cattle (Ongole Crossbred = OC) with average body weight of 78.48±7.69 kg and Fries Holland Crossbred (FHC) with body weight of 83.93±17.67 kg were kept in individual cages of 1x1.2 m and they were given four kinds of Napier Grass subtituted with SRSC of 0, 25, 50 and 75 percent of forage dry matter. Therefore, factorial experiment of 2x4 was used in this research. The dry matter ratio of forage and concentrate was maintained at 50:50% with total dry matter intake 3% of body weight. Total Volatil Fatty Acid (VFA) production were influenced by the breed of cattle and the body weight, but there was no significant effect of breed on the N-NH3. Both average of VFA (122.92±3.22 mM/l) and N-NH3 (4.14±0.4 mM/l) were still in the optimum range for rumen micoorganisme activities. Regression analysis showed that the digestibility of crude fiber had same pattern with acetate (C2), propionate (C3), butirate (C4), and valerate (C5). It can be concluded that SRSC can be used to substite fresh Napier Grass up to 75 percent of forage dry matter in the diets of male cattle both OC and FHC
