The objective ofthis study was to evaluate the effects of increasing levels of rumen-degradableprotein (RDP = 47, 52, 57 and 62%) associated to starch sources of low (corn)and high (cassava by-product (CBP) ruminal degradability on in vitroDM digestibility (IVDMD) and average daily gain of growing steers. In vitroDM digestibility was determined by using the Tilley and Terry (1963) technique.The in vitro data was submitted to analysis of variance and regressionanalysis in function of dietary RDP levels. The in vivo trial was conductedusing 32 crossbred steers (Nellore x Red Angus) averaging 275 kg of body weightand fed diets with 50:50 forage:concentrate ratio. Animals were housed in pairsand assigned to treatments according to a completely randomized design. No effectof increasing RDP levels was observed on IVDMD on diets supplemented with cornindicating no lack of nitrogen. Thus, corn digestibility appeared to be thelimiting factor on IVDMD. A significant quadratic effect was found for IVDMDby increasing the RDP levels on diets supplemented with CBP suggesting thata better synchronization between energy availability and ammonia release improvedmicrobial growth, which increased the efficiency of energy and RDP utilizationon diets with high RDP. It was not observed effect of different RDP levels associatedto a starch source rapidly degraded in the rumen on DM intake, average dailygain, and feed conversion on feedlot steers.
