A model for zinc metabolism in sheep given a diet of hay

1. Four pairs of Scottish Blackface ewes were given a basal diet of hay, providing 8 mg zinc/d, and one of four levels of supplementary Zn (0,75, 150 or 225 mg/d) continuously by intraruminal infusion. 2. 65Zn (59 microCi) was given intravenously after stabilization for 14 d and the changes in specific radioactivity (SR) of plasma and faeces were monitored for a further 10 and 13 d respectively. The faecal endogenous losses and absorption of Zn were estimated, using the principle of isotope dilution, by two methods using the whole or latter parts of SR:time curves. 3. Faecal endogenous losses were estimated by the part area method to be 4.9, 6.4, 5.1 and 6.3 +/- 0.35 mg/d at 0, 75, 150 and 225 mg supplemental Zn/d i.e. largely unaffected by Zn intake and averaging 0.11 mg/kg live weight (LW). The whole area method gave similar results. 4. Urinary excretion was negligible (probably less than 0.2 mg/d) for all sheep. 5. The amount of Zn absorbed was assumed to equal the irreversible loss of Zn under steady-state conditions and found to remain constant at 7.6 +/- 0.39 and 10.3 +/- 0.6 mg/d when calculated by "whole' and "part area' methods. Zn retention did not increase with Zn intake and homoeostasis was achieved primarily by control of Zn absorption which fell from 0.75 to 0.03 or from 0.96 to 0.05 of intake, depending on method of calculation, as intakes increased.