Seis bovinos da raça Nelore, todos machos inteiros, com idade aproximada de 12 meses e pesando em média 270 kg, serviram num experimento em gaiolas individuais, seguindo delineamento em “switch-back”, consumindo duas rações, sendo uma testemunha (sem uréia) e outra onde 1/3 do seu nitrogênio (N) total foi fornecido pela uréia em mistura com o melaço. Determinados os balanços metabólicos do N e da energia, concluiu-se que os animais quando não consumindo uréia, retiveram em média 41 g de N/dia e 15.048 kcal/dia de energia bruta e quando consumindo a ração com uréia, as retenções foram de 27 g N/dia e 16.037 kcal/dia. Houve diferença estatística significativa ao nível de 5°/o de probabilidade para os dados de N e não houve diferença estatística significativa para os balanços de energia.

Six yearling Nelore bulls averaging 270 kg, were submitted to special management during 30 days, to be used in a metabolism trial, on appropriate cages. At the same time, the animals were given a diet containing 30% of its nitrogen as urea-N.Experimental rations were balanced to supply the bovines’ requeriments (Tables II and III).Statistical method followed was the switch-back or double-reversal design, with three individuals per ration and three collection periods, each one lasting 10 days; being 3 days for adaptation of the animals to the cages and 7 days for collection of feces and urine.The total collection technique was followed. Daily amounts of feed offered during the experimental periods represented 90% of the consumption during adaptation period.Twenty-day intervals were observed between collection periods, so that the animals could make maximum utilization of the NPN.Samples of feces and urine were saved as 5% aliquots of the total daily excretion, from each animal. Ali samples were kept frozen until the end of each period; then, those samples from the same individual, were pooled and a new sample saved for chemical analyses.Calculations were made to determine metabolic balances for dry matter, nitrogen, energy, crude fiber and ether extract (Tables 1 to V), which averages and standard errors are presented on Table X.Statiscal analyses showed no significant differences between rations for dry matter digestibility. Highly significant difference was observed for nitrogen balances in g/day, with more N retained 011 ration A control, than ration B treatment (30% N as urea-N).Crude fiber study revealed significant difference for crude fiber balance with results favouring ration B treatment. With respect to energy study, no significant difference was detected for energy balance (kcal/day) or percent digestible energy.Under the condition of this trial it may be concluded that zebu cattle, ranging from 12 to 18 months old, makes no adequate use of urea-N.