EVALUATION OF THE SPOT URINE SAMPLING TECHNIQUE TO ASSESS URINARY PURINE DERIVATIVE EXCRETION IN LACTATING DAIRY-COWS

The potential of the spot urine sampling technique as an alternative t o performing a total urine collection was evaluated. Twelve multiparou s Holstein-Friesian cows were given two experimental diets in a comple te change-over design using two 14-day experimental periods. Experimen tal diets were either silage offered ad libitum with 7 kg fresh weight concentrate supplement as a single meal (SF), or a complete diet form ulated from the same ingredients with a similar forage:concentrate rat io (CD). Total urine collections were performed every 2 h on days 11 a nd 14 of each experimental period. Subsamples of urine were stored at -20 degrees C and subsequently analysed by high-performance liquid chr omatography. Daily allantoin and purine derivative (PD) excretion were highly correlated (r = 0.995, no. = 48, P < 0.001). PD and creatinine excretion during each 2-h interval depended on time of collection (PD , P < 0.001 and creatinine, P < 0.05) and on cow (P < 0.01) but were u naffected by sampling day or treatment. Diurnal variations in the mola r ratio of PD or allantoin to creatinine (PD/c and A/c, respectively) followed similar diurnal patterns as observed for PD and allantoin exc retion. The data were used to assess the error of prediction of daily mean PD/c or A/c ratios. Three spot sampling regimens (based on the co llection of four 4-h samples, three 8-h samples or two 12-h samples) a nd also on either single or 2-day urine collections were evaluated. Co llection of multiple samples within a day was more reliable than colle cting fewer samples over several days. Prediction errors were greater for SF compared with CD. Even the most intensive sampling regimen did not allow nit acceptable prediction of daily mean PD/c or A/c ratio, m inimum r values for PD/c and A/c ratios were 0.098, 0.136 and 0.547, 0 .579 for SF and CD, respectively. Furthermore, daily mean PD/c and A/c ratios proved poor predictors of daily PD and allantoin excretion (r values of 0.69 and 0.72, respectively). Total urine collection appears necessary to assess accurately daily PD excretion in dairy cows.