The effects ofcollection days on urinary excretion of creatinine and purine derivatives (PD),absorbed purines and microbial N synthesis (Nmic) were evaluated in differentcategories of Nellore cattle: heifers, steers, bulls and lactating cows. Yieldof Nmic estimated from spot urine samples was compared to that obtainedby total urine collection. Sixteen feedlot Nellore animals were assigned toa completely randomized design in a split plot scheme with treatments (two levelsof concentrate [25 or 50%] and four categories [heifers, steers, bulls and lactatingcows] 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 volumewas not affected by concentrate levels and collection days, although a significanteffect was observed for cows. Neither treatments nor collection days significantlyaffected urinary excretion of creatinine that averaged 27.1 mg/kg0.75.Similar results were observed for absorbed purines and Nmic production. Productionof Nmic estimated by spot urine sample did not differ from that obtainedby total collection. The estimation of Nmic also was not affected by concentratelevels and animal category. It was concluded that 24-h urinary collection periodis recommended for Nellore cattle and that was independent of animal category(heifers, steers, bulls and lactating cows). Therefore, spot urine samples canbe used to estimate microbial protein synthesis.
