J. Anim Sci.
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J. Anim Sci. 1986. 63:77-83.
© 1986 American Society of Animal Science
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Plasma, Rumen and Urine Pools in Urea Dilution Determination of Body Composition in Cattle1

S. J. Bartle2 and R. L. Preston2,3,

Texas Tech University, Lubbock 79409

Abstract

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.

Footnotes

1 Approved for publication by the Dean of the College of Agr. Sci. as publication No. T-5-201. Funds provided by the Thornton Endowment, Texas Tech Univ.

2 Dept. of Anim. Sci.

3 The authors acknowledge Mr. Ray McPherson for the care and feeding of the cattle; Ms. Denice Lowe for laboratory assistance and Dr. R. W. Weaver and Ms. Lisa Lucia, Dept. of Soil and Crop Sci., Texas A&M Univ., for the percent 15N excess determination.






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Copyright © 1986 by the American Society of Animal Science.