The objective of this trial was to investigate the effects of increasing dietary crude protein levels on N balance and metabolizable protein requirements for maintenance in Nellore heifers, bulls and steers averaging 16 months of age and body weight of 254.8, 285.1, and 265.6 kg, respectively. Animals were blocked by sexual category (heifers, bulls and steers) and randomly assigned to treatments in three replicated 4 x 4 Latin squares. Diets contained the following CP levels: 7, 10, 13, and 15%. Estimation of fecal DM output and duodenal DM flow was done using the internal marker indigestible ADF. Total collection of urine was conducted from day 13 to day 14 on each period. Metabolic fecal nitrogen, urinary endogenous losses and total endogenous losses were all estimated using a regression approach. No significant effect of sexual category was observed for any of the studied variables, excepting for N intake that was greater on steers than heifers. N intake, urinary and fecal N excretions, and N balance increased linearly when the dietary CP level varied from 7 to 15%. Fecal metabolic N and urinary endogenous N averaged 6.69 gN/kg of DMI and 0.133 g N/kg0.75 across diets, respectively. The net protein requirement was estimated as 0.431 gN/kg0.75 or 2.69 g of protein/kg0.75. No effect of sexual category on metabolizable protein requirements for maintenance (approximately 4.0 g/kg0.75) was observed in this trial.
bull; heifer; protein requirements; steer
