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Phosphorus Metabolism in Ponies Fed Varying Levels of Phosphorus¹

Equine Research Program and Department of Physical Biology, New York State Veterinary College and New York State College of Agriculture, Cornell University, Ithaca, New York 14850

Phosphorus metabolism was studied by balance and kinetic methods in ponies fed diets containing 0.2, 0.54 (0.25% as sodium phytate), 0.79 or 1.19% phosphorus and 0.35 to 0.54% calcium. Phosphorus retention, urinary, and total fecal excretion and the amount absorbed from the intestine were dependent on intake. However, the endogenous fecal excretion estimated by ²²P excretion, and the efficiency of absorption were unaffected by intake and averaged 9.4 mg P/kilogram body weight per day and 45%, respectively. Plasma phosphorus increased and calcium decreased when ponies were fed 1.19% phosphorus but the concentrations were unaffected by other levels of intake. Sodium phytate was utilized as efficiently as monosodium phosphate and did not alter other aspects of phosphorus metabolism. Young ponies retained similar amounts of phosphorus when fed a low calcium (0.15%) or a high calcium (1.5%) diet. The endogenous fecal excretion was the major obligatory loss of phosphorus. Assuming an average absorption of 45%, ponies require 21 mg of phosphorus/kilogram body weight per day to balance this loss. A similar estimate of the maintenance requirement was obtained by extrapolating retention to zero intake.

² Equine Research Program.

³ Department of Physical Biology.

Manuscript received 16 February 1971.