Novel methodology allows simultaneous measurement of true phosphorus digestibility and the gastrointestinal endogenous phosphorus outputs in studies with pigs

Methodology was developed for measuring the gastrointestinal endogenous pho sphorus (P) outputs and true P digestibility values in studies with piglets . Four barrows, average initial body weight 6.8 kg, were fitted with a simp le T-cannula at the distal ileum and fed four diets according to a 4 X 4 La tin square design. Four cornstarch-based diets containing four levels of P (1.1, 2.1, 3.2 and 4.3 g/kg diet) on a dry matter (DM) basis were formulate d from soybean meal (SBM). Each experimental period comprised 8 d with a 4- d adaptation and 4-d collection of ileal digesta and feces. The apparent il eal and fecal P digestibility values in SBM were affected (P < 0.05) by P l evels in the assay diets. The ileal and fecal P digestibility values increa sed from -24.8 to 37.1% and from 18.8 to 42.5%, respectively, as P contents increased from 1.1 to 4.3 g/kg DM diet. Linear relationships (P < 0.05), e xpressed as g/kg DM diet intake, between ileal and fecal outputs and dietar y inputs of P, suggested that the endogenous P outputs can be determined by linear regression analysis. The endogenous P output was higher (P < 0.05) in ileal digesta than in feces (0.86 +/- 0.09 vs. 0.31 +/- 0.06 g/kg DM die t intake). There was no difference (P > 0.05) between the true ileal (50.7 +/- 7.1%) and fecal (48.5 +/- 5.4%) P digestibility values in SBM. These re sults suggest that differences in P contents between assay diets are primar ily responsible for the large variability in apparent P digestibility value s reported within the same ingredient. Apparent digestibility values undere stimate the true digestive utilization of P by similar to 25%. True rather than apparent P digestibility values should be determined and used in diet formulation for pigs. In addition, this study shows that the gastrointestin al endogenous P output is important in whole-body P requirement and homeost asis.