Four ruminally and duodenally cannulated Holstein steers (450 kg) were fed diets with rumen degradable protein (RDP) levels of 50.0 60.0 65.0 and 70.0% associated with a high ruminal degradability starch (cassava by-product meal) were allotted to a 4 × 4 Latin square design for the evaluation of total and partial digestibility of the nutrients, ruminal ammonia concentration and pH. The acid insoluble ash was used as a marker of the duodenal and fecal flow. No effects were observed on the level of RDP on total digestibility coefficient, ruminal digestibility and intestinal digestibility coefficients of dry matter, organic matter, crude protein, neutral detergent fiber, non structural carbohydrates and gross energy. The increase of the RDP levels in the diet increased the coefficients of total and ruminal digestibility of crude protein and reduced the intestinal digestibility of this nutrient as a percentage of the digested. There was no effect of RDP levels on the ruminal pH, however, higher ammonia concentration was observed for the diets with higher level of RDP. The results indicated that the increase in the level of PDR cause a higher production of nitrogen in the ammonia form, independently of the presence of the source of high ruminal degradability starch, however the contribution of intestinal protein was similar for all the diets.
cassava by-product meal; fish meal; steers; urea
