Four, rumen fistulated crossbred (Brahman × native) beef cattle steers were randomly assigned to receive four dietary treatments according to a 2 × 2 factorial arrangement in a 4 × 4 Latin square design. Factor A was carbohydrate source; cassava chip (CC) or CC + rice bran at a ratio 3:1 (CR3:1) and Factor B was cotton seed meal level (CM); and 109 g CP/kg (LCM) and 328 g CP/kg (HCM) at similar overall CP levels (490 g CP/kg). The animals were fed 5 g concentrate/kg BW, and urea-treated rice straw (UTS) (50 g urea/kg DM) was fed ad libitum. Carbohydrate source did not affect feed intake, nutrient digestibility, blood urea nitrogen, rumen fermentation, or microbial protein synthesis; however, animals fed with CC had a higher population of total viable bacteria than the CR3:1 treatment (P<0.05). Animals that received HCM had a lower total feed intake while ruminal pH was higher than the LCM fed treatment (P<0.05). The population of total viable and cellulolytic bacteria in animals that received HCM were lower than the LCM fed treatment (P<0.05). Moreover, use of HCM in beef cattle diets resulted in lower microbial protein synthesis when compared with the LCM fed treatment (P<0.05) although efficiency of microbial protein synthesis was nonsignificantly different among treatments. Therefore, cassava chip combined with rice bran can be used in a concentrate diet for beef cattle. It is also noted that a high level of cotton seed meal in the concentrate may impact rumen fermentation and animal performance.