To increase nitrogenous compounds and improve fermentation quality of King grass (Pennisetum purpureum × Pennisetum typhoides) silage, ensiling with Manihot esculenta was tested. The treatments for silage making were: 100% king grass (KG), 25% cassava (C):75% KG, 50% C:50% KG, 75% C:25% KG and 100% C. Fresh plant materials were chopped to 1 cm length, mixed according to treatments, ensiled in laboratory silos and stored a 25°C for 60 d. After opening silos, dry matter (DM), pH, total nitrogen content (TN), protein nitrogen (PN), soluble nitrogen (SN), PN/TN, ammonia nitrogen (AN), neutral detergent fiber nitrogen of total nitrogen (NNDF/TN), and acid detergent fiber nitrogen of total nitrogen (NADF/TN) were determined. The data were analyzed according to a completely randomized design with 3 replications, significance among mixing levels was determined by Tukey test. Silage DM increased (P < 0.01) with the inclusion of cassava in the mixtures, reaching the highest values (25.95%) with 75%. The introduction of cassava significantly (P < 0.05) decreased pH levels of the silages, showing the lowest value (3.88) with 100% cassava. The TN, PN, SN, and PN/TN contents increased linearly (P < 0.05) as the percentage of cassava increased in the mixtures. Proportion of neutral and acid detergent nitrogen (NNDF and NADF) to total nitrogen were not affected by treatments (P > 0.05). A small amount of AN was detected in silage, however, there were not significant differences (P > 0.05) between 50 and 100% mixing levels of cassava. In this study, the inclusion of cassava at the rate of 25% or more, increased nitrogenous compounds and improved silage fermentation.