Evaluation of collection days and times to estimate urinary excretion of purine derivatives and nitrogen compounds in grazing Nellore cattle

The objectives of this study were to evaluate the effects of collection day and 4-hour collection times on the urinary excretion of creatinine, purine derivatives (PD), and nitrogen compounds in grazing Zebu cattle. Five Nellore heifers (300 ± 15.2 kg body weight [BW]) were used in a 5 × 5 Latin square design. Five treatments were tested: a mineral mixture supplied at 60 g/day (g/d) (control - CON); two treatments higher in salt and urea to achieve a reduced ad libitum intake of supplements: one with 39.7% crude protein (CP) on a dry matter (DM) basis (RI40) and another with 69.4% CP (RI70), both fed ad libitum; and two supplements with restricted feeding rates relative to BW: both with 20.3% of CP fed at 5 g/kg BW (BW5) or 10 g/kg BW (BW10). Each experimental period lasted 17 d (12 used for adaptation and 5 for sample collection). Six collection times at 4-hour intervals were established for each 24-hour collection day. For total urine collection, a Foley catheter was used and the collection bag attached to the animal was emptied every 4 h. Collection day, collection time, treatment or their interactions (P > 0.05) had no effect on urinary excretions of creatinine, allantoin, uric acid, and PD, nor on their ratio with creatinine. However, an interaction between collection times and treatment (P < 0.05) was observed for the excretions of urinary nitrogen and urea nitrogen, and for their ratio with creatinine. The results supported the use of a single day-collection estimate the excretion of purine derivatives and nitrogen compounds in beef cattle kept in pasture. A single urine collection for 4-hour obtained at any time during the day was adequate to estimate the purine derivatives excretion. However, using a single 4-hour urine collection to estimate the urinary excretion of nitrogen compounds in grazing Zebu cattle is not recommended.