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Manuscript received 8 January 1996. Initial reviews completed 11 March 1996. Revision accepted 13 August 1996.
Department of Animal Science, University of British Columbia, Vancouver, BC, Canada V6T 2A2 and * Center for Animal Health and Production, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA 19348
Changes in Se metabolism were studied in ewes fed hay containing normal or inadequate levels of Se. After intravenous injection of 75Se-sodium selenite, blood, feces and urine were collected at different times, and the concentrations of labeled and unlabeled Se were determined. Ewes were killed on 1, 5, 9 or 14 d after tracer injection, and tissues were obtained for determination of radioactivity and Se concentration. The data were fitted to a compartmental model using the SAAM/CONSAM computer program, and kinetic parameters and steady-state transport rates were estimated. Daily Se intake (Vi) and fecal excretion (VF) were significantly (P < 0.001) higher in the ewes fed normal hay (6.06 ± 1.09 and 3.36 ± 0.88 µmol/d, respectively) than in those fed Se-deficient hay (0.64 ± 0.18 and 0.26 ± 0.15 µmol/d). The net absorption (Va) of Se was significantly higher in ewes fed normal hay [3.19 ± 0.82 µmol/d by the balance method, Va = Vi 
(VF Vf) (Vf = endogenous fecal Se) and 1.05 ± 0.38 µmol/d by using the model (plasma entry rate, U(1)] than in those fed hay deficient in Se [0.57 ± 0.33 µmol/d (balance) and 0.28 ± 0.08 µmol/d (model)]. The efficiency of absorption (
= U(1) ÷ Vi) was significantly higher (0.46 ± 0.19) in ewes fed Se-deficient hay than in those fed normal hay (0.18 ± 0.09). Simultaneous fitting of the tracer data of both the groups showed that changes in hepatic extraction and urinary and fecal excretion were sufficient and necessary to account for the kinetic differences observed between treatments.
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