Prediction Models for Standardized Total Tract Digestible Phosphorus in Swine Diets

The objectives of the present study were to develop equations for predicting the standardized total tract digestible (STTD) P based on total P and phytate-P and supplemental phytase concentrations in swine diets and to evaluate the accuracy of the prediction models for STTD P using recently published data. A total of 414 data from 65 experiments that used supplemental phytase and determined P digestibility were employed to develop equations for predicting STTD P in swine diets. The CORR procedure of SAS was used to determine correlation coefficients among variables and the NLMIXED procedure was used to generate prediction equations. The STTD P was positively correlated with total P (r = 0.73; P < 0.001), phytate-P (r = 0.21; P < 0.001), and supplemental phytase (r = 0.36; P < 0.001) concentrations. Three equations for predicting the STTD P were: in the linear model, STTD P, g/kg as-fed = 0.244 + 0.660 × total P – 0.432 × phytate-P + 0.224 × phytate-P × phytase with Akaike information criterion (AIC) = 503.5 and Bayesian information criterion (BIC) = 523.6; in the quadratic model, STTD P, g/kg as-fed = 0.179 + 0.662 × total P – 0.422 × phytate-P + 0.286 × phytate-P × phytase – 0.011 × (phytate-P × phytase)² with AIC = 498.8 and BIC = 523.0; in the exponential model, STTD P, g/kg as-fed = 0.214 + 0.664 × total P – 0.444 × phytate-P + 0.581 × phytate-P × (1 – e^{–0.603 × phytase}) with AIC = 498.6 and BIC = 522.8, where total P range, 1.05 to 7.90 g/kg as-fed; phytate-P range, 0.40 to 6.18 g/kg as-fed; and phytase range, 0 to 2.22 FTU/g as-fed. According to the regression analysis for testing the accuracy of the developed quadratic model using recently published 42 STTD P data, the intercept (0.014 ± 0.041; P = 0.739) representing the mean bias and slope (−0.009 ± 0.051; P = 0.865) representing the linear bias were not different from 0. When validating the exponential model with the same method, the intercept (0.021 ± 0.038; P = 0.582) and slope (0.004 ± 0.048; P = 0.935) were also not different from 0. In conclusion, the equations suggested herein can fairly accurately predict STTD P concentrations using total P, phytate-P, and supplemental phytase concentrations as independent variables in swine diets.