The work was carried out to evaluate the plasma N-urea concentration (NUP), the urea fractional excretion (UFE) and the creatinine (CE) and urea (EU) excretions for several experimental conditions; to establish the relation among NUP, the urea fractional excretion and the urea excretion with the dietary crude protein (CP) and NDF percentage; and to evaluate the urinary endogenous losses, by means of the creatinine excretion determination. In the experiments I and II, diets with 25.0, 37.5, 50.0, 62.5, and 75.0% of concentrate formulated were used to have different protein levels according to the dietary energy, or similar protein levels (12%CP), respectively. In the experiment III, concentrate level was fixed (40%), and the forage cut age was variable. In the experiment IV, diets were constituted on 20.0, 32.5, 45.0, 57.5, and 70.0% of concentrate, with approximately 12% CP. An experimental 5 x 5 latin square design, except for the experiment III (4 x 4), was used. From the total urine collected in 24 hours, the urea and creatinine excretions were determined. For the experiments I and III, the NUP concentration linearly changed as the dietary CP and NDF percentage increased. The fractional urea excretion and the urine excretions varied the same way for all treatments. The creatinine excretion was not influenced by the dietary CP content being proportional to the body weight for all experiments, with an average value of 27.36 mg/day/LW, and it could be used to estimate the endogenous losses of nitrogenous compounds.
creatinine; steers; urea
