Productive performance, during the growing and fattening phases, of Nellore bulls fed diets with different concentrate and protein levels

Thirty six Nellore bulls, with 240 kg initial live weight (LW), were used. Four were reference bulls and the remaining were distributed in eight treatments, with four different concentrate levels in the diets (20; 40; 60 and 80%) and two crude protein (CP) levels (15 and 18%), in dry matter basis. The dry matter (DMI), organic matter (OM), CP, neutral detergent fiber (NDF) and total digestible nutrients (TDN) intakes were determined, as well as the average LW gain (LWG), empty body weight gain (EBWG), carcass gain (CG) and the feed conversion (FC). The DMI in the growing phase, expressed in kg/day, was not influenced by the concentrate level, presenting mean value of 7.06. But the DMI, expressed as a percentage of the LW (% LW) and in grams for unit of metabolic size (g/kg(0.75)), in this phase, demonstrated a quadratic behavior in relation to the concentrate level, with maximum consumptions of 2.59% LW and 107.32 g/kg(0.75), with 42.60 and 42.52% of concentrate, respectively. In the fattening phase, the concentrate levels did not influence the DMI, in any of the expressed ways, with mean values of 7.16 kg/day; 1.80% LW and 80.64 g/kg(0.75). The increase in the diet CP level, from 15 to 18%, caused a significant increase in the growing phase DMI, but it did not influence the consumption in the fattening phase. The diets concentrate levels did not influence the OM, CP and TND intakes, in the growing and fattening phases. The average values of OM, CP and TND intake, expressed in kg/day, were 6.50; 1.18 and 5.52, respectively, during the growing phase; and 6.89; 1.20 and 5.59, respectively, during the fattening phase. The NDF consumption, expressed in kg/day and in % LW, had a decreasing linear behavior with the concentrate level increase. LWG, EBWG, CG and FC were not influenced by the concentrate levels of the diet, independently of the studied phase. In the growing phase, the mean values for LWG, EBWG, CG and FC were 1.15; 1.13 and 0.75 kg/day and 6.29, respectively; and, in the fattening phase, they were 1.05; 1.00 and 0.71 kg/day and 8.02, respectively. The diet CP level had a positive influence on the LWG in both phases. The diet with 18% of CP provided, to the bulls, 22 and 22.6% more LWG in the growing and in the fattening phases, respectively, compared to the diet with 15% CP. The highest concentrate levels presented the smallest production cost during the growing phase, and the levels of 40 and 60% of concentrate presented the smallest cost in the fattening phase.