The objectives of the present work were to estimate the microbial protein production using the total purine derivatives (PD) excretion, to compare the PD and the urea excretions, obtained from spot urine collection with that observed in the 24 hours collection and to evaluate the plasma and milk N-urea and urea urinary excretion. Sixteen Holstein lactating cows were assigned to four 4X4 Latin Square with 3-wk periods in accordance with the lactation period. The four experimental isoproteic diets were formulated to contain 60% of corn silage and 40% of corn meal, soybean meal, urea and mineral salt mixture as dry matter basis. Crescent levels of urea in the concentrate 0.0, 0.7, 1.4, and 2.1% corresponded to the crude protein of 2.22, 4.18, 5.96 and 8.09% of non protein nitrogen based compounds (NPN), respectively. The urine collection was also estimated with urine samples obtained four hours post fed (spot), by the relation of the daily mean creatinine excretion and the urine spot creatinine concentration. The urinary volume and PD excretion and urea estimated through the urine spot did not differ from that obtained from 24 h urine collection. The N-Microbial production estimated and obtained presenting maximum values of 198.05 and 196.96 g/day with the NPN levels of 5.33 and 4.44%, respectively. The milk allantoin concentration linearly decreased as the NPN increased in the diet and represented 4.5% of total PD excretion. The creatinine excretion was not influenced by the dietary NPN levels (23.41 mg/kg LW), while the estimated and obtained urea excretion, the plasma and milk urea and N-urea concentration linearly increased as the NPN increased in the diet. The plasma and milk N-urea concentration of 19-20 mg/dL and 24-25 mg/dL, respectively, represented the limit values from that would occur nitrogen compounds losses.
creatinine; purine derivatives; non protein nitrogen compounds; urea; lactating cows
