AN EVALUATION OF VARIOUS RESPONSE CRITERIA IN ASSESSING BIOLOGICAL AVAILABILITY OF PHOSPHORUS FOR BROILERS

The relative bioavailability of P from seven sources was determined in relation to a standard dicalcium phosphate dihydrate (CaHPO4 . 2H(2)O) in a 21-d assay involving 1,320 broiler male chicks using several response criteria. The seven sources (Lucaphos-48, Lucaphos-40, Rukana, Cefkaphos-N, phosphoric acid, monocalcium phosphate monohydrate, or Biophos) were added to the basal diet (.40% total P and 1.10% Ca) at levels to supply .05, .08, .12, .17, .23, and .32% P. Two additional levels (.44 and .66%) of P from dicalcium phosphate dihydrate were included for the standards. The criteria selected to evaluate included tibia ash, tibia specific gravity, tibia shear force, toe shear force, and metatarsal shear force. Other criteria: weight, length, diameter, and volume of tibia; weight, volume, and specific gravity of metatarsus; and weight, volume, and specific gravity of toe were not selected because their response to increasing P levels were inconsistent and quite variable. Nonlinear (asymptotic and sigmoidal) regression equations were fitted to the data for the selected criteria because they were a better fit of the data than linear equations. The ratios of regression coefficients were used to determine the bioavailability of various test phosphates relative to the reference standard. The results indicated that the response criteria used for the determination will considerably influence the relative bioavailability estimates of a P source. Body weight gain and toe ash percentage were found to be an equally or a more sensitive criteria for assessment of P availability than tibia ash. Tibia specific gravity, tibia shear force, toe shear force, and metatarsal shear force were of limited value as response criteria in P bioavailability assays based on standard error and difference required for significance.