This work was carried out to evaluate the effect of four dietary urea levels (0, 0.65, 1.3, and 1.95% in dry matter - DM) on ruminal pH and ammonia-N, plasmatic urea nitrogen, urea fractional excretion, creatinine and urea excretions, as well as to evaluate endogenous urinary losses, by means of creatinine excretion, in steers of four genetic groups (Holstein, ½ Holstein-Guzera, ½ Holstein-Gir and Zebu). Animals were assigned to four 4 <FONT FACE=Symbol>´</FONT> 4 latin squares (genetic groups), with four animals, four experimental periods of 14 days each one and four treatments (rations). The ruminal fluid collections to determine pH and ammonia nitrogen (N-NH3) concentrations was performed before feeding and 2, 4, 6 and 8 hours post feeding. During the 24-h urine collection, 4 hours after feeding, blood was collected and, after centrifugation, plasma was obtained. In the urine and plasma samples, the urea and creatinine concentrations were determined. Ruminal pH showed the similar pattern for the genetic groups and was affected by the increasing urea levels in the diet. The ruminal ammonia concentration were affected by the increasing urea levels in the diet for the Holstein and crossbred. The concentration of plasmatic urea nitrogen linearly increased as urea level in the diet increased. Urea fractional excretion decreased linearly by the increasing urea level in the diet. Urea urinary excretion did not differ among Holstein, but showed higher values in relation to Zebu. Creatinine excretion was affected neither by the genetic group nor by the increasing dietary urea levels and showed average value of 27.76 mg/kg of LW. It is recommended that endogenous urinary losses of nitrogen compounds can be estimated from the creatinine excretion.
ruminal ammonia; creatinine; steers; ruminal pH; urea
