The effects of collection days on urinary excretion of creatinine and purine derivatives (PD), absorbed purines and microbial N synthesis (Nmic) were evaluated in different categories of Nellore cattle: heifers, steers, bulls and lactating cows. Yield of Nmic estimated from spot urine samples was compared to that obtained by total urine collection. Sixteen feedlot Nellore animals were assigned to a completely randomized design in a split plot scheme with treatments (two levels of concentrate [25 or 50%] and four categories [heifers, steers, bulls and lactating cows] included in the plots (2 x 4 factorial arrangement) and collection days (six days) in the sub-plots. No significant interactions among concentrate levels, animal categories and collection days were observed. In addition, urinary volume was not affected by concentrate levels and collection days, although a significant effect was observed for cows. Neither treatments nor collection days significantly affected urinary excretion of creatinine that averaged 27.1 mg/kg0.75. Similar results were observed for absorbed purines and Nmic production. Production of Nmic estimated by spot urine sample did not differ from that obtained by total collection. The estimation of Nmic also was not affected by concentrate levels and animal category. It was concluded that 24-h urinary collection period is recommended for Nellore cattle and that was independent of animal category (heifers, steers, bulls and lactating cows). Therefore, spot urine samples can be used to estimate microbial protein synthesis.
uric acid; allantoin; nutrition; ruminants
