This research was carried out, using 15 dairy cows (Holstein x Gyr) full fed isoproteic diets with 60% silage and 40% concentrate in the dry matter (DM) basis and 0, 0.7, 1.4, and 2.1% urea correspondent to 2.08, 4.01, 5.76, and 8.07% of crude protein levels in the form of non protein nitrogen compounds (NNP), with the objectives to a non invasive methodology, to estimate the microbial protein production, using the total excretion of purine derivatives (PD); to evaluate the concentrations of creatinine, urea and N-urea in the plasma and in the milk; and to compare the PD and urea excretions obtained from total urine and spot collections. The animals averaging initial live weight of 511.8 kg were randomized allotted to the treatments. The experimental period last 90 days for each cow, starting immediately after calving. The results were interpreted by variance and regression analyses. The urine samples were obtained from 24-h collections or from urine sample obtained, approximately, four hours post feeding (spot urine), while the blood samples were obtained four hours post feeding offer. The urea and N-urea concentrations in the plasma and in the milk, as well as the creatinine excretion, that showed average value of 23.60 mg/kg LW, were not affected by the addition of increasing NNP levels to the diet. The urinary excretions of urea, allantoin, uric acid and PD, the absorbed purines and the microbial N production were not influenced by dietary NNP levels. The daily estimated excretions of urea, allantoin and uric acid followed the same pattern obtained by the 24-h collection. Urine spot sample was satisfactory to estimate the PP excretion and, subsequently, the microbial N production.
creatinine; purine derivatives; milk; plasma; urine
