Four ruminally-cannulated non-pregnant Brahman beef cows (body weight (BW) = 430 kg) were used to study the effects of yeast-fermented cassava roots (YEFECAR) as the sole source of protein on dry matter (DM) intake and nutrient digestibility. Animals were allotted to groups fed YEFECAR at levels: 0, 20, 30, 35% DM, according to a 4 × 4 Latin square design. Cows were fed rice straws (as a roughage) ad libitum; concentrate treatments (1.5% BW per day) were offered twice daily. It was indicated that the use of YEFECAR in a concentrate diet reduced feed intake (P < 0.05) but had no effects on nutrient digestibility, rumen ecology (pH, NH3-N), rumen volatile fatty acid concentration, rumen bacterial population and microbial protein. Accordingly, it can be concluded that the use of YEFECAR as the primary source of protein at 20% DM or higher in concentrate feed can lower the feed intake, which may subsequently affect livestock yield. To ensure optimal utilization of YEFECAR as the main source of protein in ruminant feed further studies on increasing the sulphur content in yeast medium prior to the production of YEFECAR are needed.