Plasma, rumen and urine pools in urea dilution determination of body composition in cattle.

To determine if urea diffuses into reticulo-ruminal water (RRW) during urea dilution estimation of body composition, four 450-kg heifers were infused intravenously with a solution containing 65.05 g urea plus .95 g 15N-urea, after a 20-h removal of feed. Blood, urine and rumen fluid were collected before infusion and at various times for 120 min after infusion. Plasma 15N clearance was described by a two-pool model. Plasma and urine 15N levels equilibrated within 12 min post-infusion and then declined at similar rates, suggesting that renal clearance is a major component of the second pool. Rumen fluid contained no urea and rumen NH3-N did not increase during the study. Rumen fluid and plasma 15N did not equilibrate over the time studied (rumen fluid 15N/plasma 15N = .07 and .17 at 12 and 120 min after infusion, respectively). Therefore, urea dilution at 12 min overestimates empty body water only by the volume of urine produced during this time; RRW influences urea dilution estimation of body composition only as a component of live weight.