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Procedural and Mathematical Considerations in Urea Dilution Estimation of Body Composition in Lambs¹

Texas Tech University, Lubbock 79409 and University of Nebraska, Lincoln 68583

Abstract

Two experiments (n = 46 and 56, respectively) were conducted to evaluate urea dilution as an estimator of body composition in lambs and to address certain procedural and mathematical considerations in this technique. In Exp. 1, 14 blood samples were taken over 240 min after urea infusion. The equation describing the urea clearance curve was: PUN = 9.7e^{–.1727(min)} + 10.4e^{–.0021(min)}, pools 1 and 2) respectively (r² = .99, P < .001 individual lamb effects removed). In the combined experiments, urea space (US) was related to percentage of empty body water (PEBH₂O) by the equation 31.7 + .471 US (empty body weight basis; r² = .56, P < .001). The regression equation indicates that the US-PEBH₂O relationship in lambs is different from that reported in cattle, even though urea clearance kinetics are similar. Although the prediction equations appeared to be biologically valid, considerable error was associated with the composition estimates. The PEBH₂O was predicted as well by live weight (r² = .69; SE_{y · x} = 3.0) as by US in these experiments. The two-sample method (T12 minus T0) to determine the change in marker concentration was shown to be related more closely (r² = .56) to PEBH₂O than the standard multisample extrapolation to T0 method (r² = .0 and .38 for pools 1 and 2, respectively). An equilibration time of 9 to 12 min provided the best estimate of body composition in lambs. The results demonstrate the importance of accurate sample timing and precise plasma urea-N determination and indicate that the US procedure may require modification for use in lambs.

¹ Approved jointly for publication by the Dean of Agric. Sci., Texas Tech Univ., as publication No. T-5-236 and Agric. Res. Div., Univ. of Nebraska, as Journal Series No. 8427. Funds were provided in part by the Thornton Endowment, Texas Tech Univ.

² Dept. of Anim. Sci., Texas Tech Univ.

³ Current address: Koers Consulting Service, Inc., Salina, KS 67401.

⁴ Dept. of Anim. Sci., Univ. of Nebraska.