Labile nitrogen reserves and plasma nitrogen fractions in growing cattle

The existence and magnitude of labile nitrogen (N) reserves were studied in growing cattle using a dietary N depletion-repletion technique. Blood parameters and urinary N excretion patterns were monitored. Blood hematocrit (Ht) did not respond to reduced ingested N until after week 3 of depletion at which time it began to fall. About 3 weeks of repletion regime was required before Ht values increased again. Plasma protein and albumin decreased from 6.05 and 2.70 g during standardization to 5.44 and 2.44 g/100 ml after 5 weeks of depletion and did not approach predepletion levels until week 6 of repletion. Plasma urea N decreased from 16 mg in standardization to 0.08 mg/100 ml in depletion and required 3 weeks of repletion treatment to attain a peak of 18 mg/100 ml. Labile N reserves were determined by integrating the areas from total urinary N excretion curves obtained during depletion and repletion periods. On a live body weight basis labile N represented 5.6% of total body N. When computed on an empty body weight basis (does not include weight of gastrointestinal tract), labile N totalled 6.0%. The magnitude of labile N stroes as a percentage of total body N was 44% greater in steers with a mean body weight of 280 kg compared with animals weighing 144 kg.