ABSTRAK. Penelitian bertujuan untuk mengetahui fermentabilitas ransum berbasis jerami padi dan kaliandra yang mengandung konsentrat terfermentasi (in vitro). Penelitian dilakukan secara eksperimental dengan rancangan acak lengkap yang terdiri dari 4 perlakuan dan 5 ulangan. Data dilakukan sidik ragam dan uji Duncan. Ransum perlakuan adalah P0 = 39% jerami padi + 21% daun Kaliandra + 40% konsentrat, P1 = 39% jerami padi + 21% daun Kaliandra + 40% konsentrat fermentasi, P2 = 44% jerami padi + 21% daun Kaliandra + 35% konsentrat fermentasi, P3 = 49% jerami padi + 21% daun kaliandra + 30% konsentrat fermentasi. Hasil menunjukan bahwa bakteri dan protozoa rumen menghasilkan populasi yang tidak berbeda nyata, sedangkan konsentrasi asam lemak terbang dan amonia berbeda nyata (P<0,05). Ransum P1 menghasilkan konsentransi asam lemak terbang yang nyata (P<0,05) paling tinggi (148,80 mM) dengan amonia yang paling rendah (5,53 mM). Sementara itu, konsentrasi amonia tertinggi (P<0,05) diperoleh pada perlakuan P0 sebesar 7,54 mM, sebaliknya menghasilkan konsnetrasi asam lemak terbang paling rendah. yaitu 5,35 mM. Kesimpulan ransum dengan komposisi 39% jerami padi, 21% kaliandra, 40% menghasilkan fermentabilitas yang terbaik. Kata kunci : Fermentabilitas, jerami padi, Kaliandra, konsentrat terfermentasi, rumen ABSTRACT. This study aims to evaluate the fermentability of rice straw and Calliandra leaves-based rations supplemented fermented concentrates (in vitro). The research was conducted experimentally with a completely randomized design consisting of 4 treatments and 5 replications. The data were analyzed by variance and Duncan's test. The treatment rations were P0 = 39% rice straw + 21% Calliandra leaves + 40% concentrate, P1 = 39% rice straw + 21% Calliandra leaves + 40% fermented concentrate, P2 = 44% rice straw + 21% Calliandra leaves + 35% fermented concentrate, P3 = 49% rice straw + 21% Calliandra leaves + 30% fermented concentrate. The results showed that the treatment had no significant effect to rumen bacteria and rumen protozoa populations, while the concentrations of volatile fatty acids and ammonia were significantly different (P<0.05). Ration P1 produced the highest (P <0.05) concentration of volatile fatty acids (148.80 mM) with the lowest (P<0.05) ammonia (5.53 mM). Meanwhile, the highest ammonia concentration (P <0.05) was obtained in the P0 treatment of 7.54 mM, on the other hand it resulted in the lowest (P <0.05) concentration of volatile fatty acids, namely 5.35 mM. Conclusion, that rations with a composition of 39% rice straw, 21% Calliandra, 40% fermented concentrate yield the best fermentability.