# Additivity of Standardized Total Tract Digestible Phosphorus in Mixed Diets and the Influence of Phosphorus Requirement Expressions in Diet Formulations on Phosphorus Excretion in Pigs

## TL;DR

This study shows that using standardized total tract digestible phosphorus in pig diets gives more accurate results and reduces phosphorus excretion.

## Contribution

The study demonstrates the additivity of standardized total tract digestible phosphorus in mixed diets and its impact on phosphorus excretion in pigs.

## Key findings

- Measured ATTD phosphorus in mixed diets was higher than predicted, showing non-additivity.
- STTD phosphorus values were additive across ingredients in mixed diets.
- Pigs fed ATTD-based diets excreted more phosphorus than those fed total or STTD-based diets.

## Abstract

Phosphorus is a key nutrient for bone development and biochemical reactions. An accurate estimation of phosphorus bioavailability is essential to meet the physiological needs of pigs. The apparent total tract digestibility (ATTD) of phosphorus is calculated based on the intake and fecal output of phosphorus in pigs, and the standardized total tract digestibility (STTD) of phosphorus additionally accounts for basal endogenous losses of phosphorus. In this study, we tested the additivity of digestible phosphorus expressions in mixed diets containing wheat and soybean meal fed to pigs, and we compared phosphorus excretions among swine diets formulated based on total, ATTD, or STTD phosphorus. The measured ATTD of phosphorus in the mixed diet was greater than the predicted value, indicating that ATTD phosphorus values were not additive. However, the measured and predicted STTD values did not differ, demonstrating that the STTD phosphorus values are additive across the ingredients. Pigs fed diets formulated based on ATTD phosphorus excreted more phosphorus than those fed diets formulated based on total phosphorus or STTD phosphorus. Overall, the present results indicate that STTD phosphorus provides a more accurate and additive measure of biologically available phosphorus in mixed diets, and the use of STTD phosphorus for diet formulations may reduce phosphorus excretion from pigs.

This study aimed to test the additivity of digestible phosphorus (P) in mixed diets fed to pigs and to investigate the effects of P expressions in diet formulations on P excretion from pigs. In experiment 1, eight castrated male pigs (30.5 ± 1.5 kg) were allotted to a replicated 4 × 4 Latin square design. Three experimental diets contained wheat, soybean meal, or both as the sole P source, and a P-free diet was prepared to determine the basal endogenous losses of P. In experiment 2, twenty-four castrated male pigs (18.1 ± 0.7 kg) were allotted to a completely randomized design. Three experimental diets were prepared based on the total, apparent total tract digestible (ATTD), or standardized total tract digestible (STTD) P. In experiment 1, the measured ATTD of P in the mixed diet was greater (p < 0.05) than the predicted values, but no difference was observed in the STTD of P in the mixed diet. In experiment 2, the daily P output of the pigs fed the diet based on ATTD P was greater (p < 0.05) than that based on the total or STTD P. Overall, the STTD P provides a more accurate and additive measure of biologically available P in mixed diets for pigs, and the use of STTD P in formulations may reduce P excretions.