Bioavailability of phosphorus in commercial phosphate supplements for turkeys

Forty-seven commercial phosphorus supplements were obtained from feed manufacturers and subjected to biological phosphorus availability assay using the turkey poult. Bone ash was the response criterion. Compared to a reference source of commercial mono/ dicalcium phosphate (assigned 100), 7 other mono/ dicalcium phosphates averaged 97.1 (range 88.6 to 110.0), 20 dicalcium phosphate sources averaged 90.6 (76.3 to 104.8), and 20 defluorinated phosphate sources averaged 78.4% (68.6 to 89.6) availabilities. Combining equal parts of all sources of each type of phosphorus supplement and using purified grade calcium phosphate monobasic monohydrate as a defined standard, the commercial mono/ dicalcium phosphates, dicalcium phosphates, and defluorinated phosphates averaged (with 95% confidence limits) 93.6 +/- 7.6, 88.3 +/- 8.2, and 70.2 +/- 7.2% phosphorus availability in a slope ratio bioassay. Calculating the requirement intercepts utilizing the blended phosphorus supplements indicated apparent requirements using mono/ dicalcium phosphate, dicalcium phosphate, and defluorinated phosphate of .74, .76, and .83% total phosphorus in the diet, respectively. Considering 1) the variability in availability of the sources, 2) variation in feed mixing, 3) the National Research Council requirement of .80%, and 4) that newly hatched poults are completely dependent on one batch of prestarter feed during 0 to 10 days of life, it is suggested that in certain instances field rickets could result from deficiency of dietary phosphorus.