Influence of dietary calcium on phosphorus absorption and excretion and on phosphorus-33 distribution in laying hens.

An experiment was conducted using isotope-dilution and comparative balance techniques to estimate urinary and fecal P excretion of laying hens fed different levels of dietary Ca. Two levels of dietary Ca (3.46 and 4.2%) were fed to eight hens for 30 days. After 30 days, 50 microCi of the radioisotope 33P was injected intramuscularly to label endogenous P. On the 2nd day after 33P dosing and at 1 h postoviposition, plasma, liver, kidney, femur bone, whole egg, ileum, ileal digesta, and excreta samples were collected from each hen. Results showed a favorable effect of increasing dietary Ca consumption (2.91 vs 3.57 g/hen per day): femur ash increased (P less than .08), excreta P decreased (P less than .03), and urinary P decreased (P less than .06). The P content of ileal digesta was not affected by dietary Ca intake, but excreta P was greater for hens consuming less Ca, indicating that, during the collection period, excretion of P in urine was increased by the low Ca diet. Endogenous P secretions constituted less than 1% of the P in ileal digesta and excreta samples and this proportion was not changed by dietary Ca consumed.