Níveis de proteína em dietas de bovinos. 4. Concentraçoes de amônia ruminal e uréia plasmática e excreçoes de uréia e creatinina

The objective of this work was to determine the ruminal ammonia N concentration (RAN), plasma urea N (PUN), and urinary excretion of urea (UEX) and creatinine (CEX) relationship in Zebu steers fed diets ad libitum with different crude protein levels. Four Zebu cattle, averaging 282.6 kg liveweight and fitted with carmulae in the rumen, abomasum, and ileum, were assigned to a 4 x 4 Latin square design, except for PUN where a split-plot design was used with crude protein levels as the whole plot and the time of blood sampling as the split plot. Rations containing 45 percent concentrate and 7, 9.5, 12, and 14.6 percent CP were offered ad libitum once a day. Urine samples were obtained in a 24-h collection period, whereas blood samples were obtained immediately before feeding and at 2, 4, 6, and 12 hours after feeding. Fractional excretion of urea was obtained from the urea and creatinine clearance ratio. RAN increased linearly with PUN. There was an interaction between collection time and dietary CP levels for PUN. Dietary CP levels affected linearly the mean PUN and fractional UEX. Dietary CP levels did not affect CEX, whereas UEX was quadratically related to the levels of dietary CP. The fractional UEX could serve as a parameter for estimating the nutritional status of cattle, and the mean PUN of 13.5 -15.5 mg/dL probably would delineate a threshold for protein losses in steers fed diets with 45 percent concentrate and 62.5 percent TDN. (RA)