Estimation of New Phosphorus Utilization by the “Slope” Method

Abstract

A mathematical model for the estimation of phosphorus (P) availability was derived and tested in 3 separate trials. This method is based on the determination of the regression slope of tibia P as a function of total P intake, under conditions of controlled feeding. Carcass P-to-tibia P was found to be 19.6 ± 1.0. The value of the regression slope multiplied by this factor and 100 yields the percentage of net phosphorus utilization (% NPU). The latter allows for the endogenous P excretion, in contrast with percentage of apparent retention. The % NPU for reagent grade dicalcium phosphate was 94.1, 98.0 and 78.4, respectively, in 3 trials. Defluorinated rock phosphate, soft phosphate and reagent grade tricalcium phosphate were 95, 76 and 92%, respectively, as available as reagent grade dicalcium phosphate. Feed grade dicalcium phosphate was as available as reagent grade dicalcium phosphate. Plasma inorganic phosphorus was determined to be a useful measure for estimation of the relative P availability. Under ad libitum feeding conditions, total P requirement level was found to be 0.65, 0.65 and 0.66% for maximal growth, feed efficiency and tibia P, respectively. The Ca-to-P ratio of bone decreased with P supplementation. The relationship between feed efficiency and dietary P is discussed.