This experiment was conducted to investigate the effect of various protein sources on digestibility, rumen fermentation, milk yield and milk composition in dairy cows. Four Holstein Friesian native crossbred cows in early lactating were randomly assigned according to a 4x4 Latin square design. The dietary treatments containing different protein sources in concentrate diets were soybean meal (SBM), cassava hay (CH), Leucaena leucocephala (LL) and yeast-fermented cassava chips (YEFECAP), with ad libitum intake of urea-treated rice straw. Digestibility of DM, OM, NDF and ADF was not different among treatments (P>0.05) while CP digestibility was highest (P<0.05) in CH and YEFECAP supplemented groups. Ruminal NH3-N and BUN concentrations varied among protein sources and were highest in SBM and LL fed groups (P<0.05). Ruminal total volatile fatty acid (VFA) and propionic acid were found highest in cows receiving CH and YEFECAP (P<0.05). Ruminal fungi, proteolytic and cellulolytic bacteria were highest when YEFECAP was supplemented. Milk fat and milk protein were significantly increased (P<0.05) in cows fed with CH and YEFECAP. Based on this study, it was concluded that providing CH or YEFECAP as protein source in concentrate diets could improve rumen fermentation and milk production in lactating dairy cows fed on rice straw
