Nitrogen metabolic evaluation of grazing bulls receiving concentrate supplementation in the finishing phase

The objective of this study was to evaluate the effect of concentrate on the nitrogen metabolism of grazing beef cattle in the finishing phase. The experiment was conducted between March and April, 2014. Eight bulls, Nellore, with an average weight of 322.1 ? 43.55 kg and age of 15.0 ? 1.35 months, housed in Urochloa brizantha cv Piat? pastures, were used, distributed in four 2 x 2 Latin square design, simultaneous two by two, with two treatments alternating between the experimental units, in four periods. The treatments included concentrated Lipomax AC^(R), supplied in an amount of 0.5% of body weight per day; or mineral supplement Real H 650^(R) (both from Real H, Campo Grande, MS), supplied ad libtum. On the 14^th day of each period, around four hours after supplied the supplement, samples of blood and spot samples of urine (10 mL, spontaneous) were taken from the animals. After collection, urine samples were diluted in 40 ml of 0.036 N H₂SO₄ and frozen at - 20?C for further analysis of creatinine, N-urea (Nurur) and total N (Ntotur). Blood samples were collected immediately after urine collection and centrifuged for further analysis of serum urea. Data were submitted to an analysis of variance according to a four Latin Square 2 x 2, simultaneous two by two, design. Comparing the averages for "t" test at 5%, using the PROC GLM of SAS v. 9.4. The level of urea nitrogen in the blood of animals receiving only mineral supplement was 23.54 mg dL^-1, lower (P<0.05) than those receiving concentrated supplement (29.24 mg dL^-1). The use of concentrated increased (P<0.05) the amount of Nurur (45.26 vs. 56.39 g d^-1 in animals receiving mineral or concentrate, respectively). The lowest amount of N-urea in the blood and urine of animals receiving mineral supplement may be associated with improved metabolic utilization of the N from the diet. The concentrate supplementation did not affect (P>0.05) the amount of Ntotur (93.01 g d^-1 in bulls supplemented with concentrates and 83.45 g d^-1 in that supplemented with minerals). The supply of concentrate supplement to grazing bulls did alter the metabolic of N usage in the finishing phase.