Phosphorus homeostasis in sheep. II.* Influence of diet on the pathway of excretion of phosphorus

Abstract

Urinary phosphorus excretion by sheep given diets of coarse or finely ground roughage to vary their salivary flow rates, was studied. A 3 x 3 Latin square design was used in which three Merino ewes with rumen fistulas were given nutritionally complete diets in which the roughage component consisted of oat hulls (diet A), finely ground oat hulls (0.8 mm screen) (diet B), and finely ground oat hulls with added polyethylene flakes (diet C). Daily rumination time was used as an index of relative saliva flow rate and varied from 7.59?0 .47 and 6.83?1.12 hr/day for diets A and C respectively, to 1.92?0.39 hr/day for diet B. Similar changes were observed for the daily number of chews. When the sheep were given diet B, the pH of the rumen fluid, the concentration of inorganic phosphorus in the rumen fluid and the total inorganic phosphorus content of the rumen were reduced relative to diets A and C. Each of these effects points to a reduced rate of saliva flow. The inorganic phosphorus levels of the plasma were unaffected by the dietary treatments except that in the 6 hr following feeding, the levels fell for treatments A and C, but rose for treatment B. Urinary phosphorus excretion, which was significantly increased by diet B, was 356.6, 472.7 and 373.5 mg/day for diets A, B and C respectively. Urinary and faecal excretions of phosphorus were significantly correlated (r = –0.86) as were the urinary excretion and rumen content of phosphorus (r = –0.88). The data are discussed in relation to the role of salivary secretion of phosphorus in the balance between faecal and urinary excretion of phosphorus in sheep. _________________ *Part I, Aust. J. Agric. Res., 25: 475-83 (1974).