IN VITRO MINERAL SOLUBILITY In vitro versus in situ evaluation of the effect of phytase supplementation on calcium and phosphorus solubility in soybean and rapeseed meal broiler diets

1. In vitro assays provide a sensitive and economic tool to evaluate dietary effects, but have limitations. In this study, the effect of phytase supplementation on solubility, and presumed availability, of calcium (Ca) and phosphorus (P) in soybean meal (SBM) and rapeseed meal (RSM) based diets was evaluated both in situ and by a 2-step in vitro digestion assay that simulated the gastric and small intestine (SI) phases of digestion. 2. Comparison of the in vitro findings to in situ findings was used to evaluate the in vitro assay. Ross 308 broilers (n = 192) were fed one of 6 SBM or RSM diets supplemented with 0, 500 or 5000 FTU/kg phytase from 0 to 28 d. The 6 diets and raw SBM and RSM were exposed to a two-step in vitro assay. Ca and P solubility and pH in the gizzard and jejunal digesta and in the gastric and SI phase of in vitro digestion were measured. 3. Both in vitro and in situ analysis detected that Ca solubility was lowest (P < 0.05) when diets were supplemented with 500 FTU/kg phytase, compared to the control diets and diets supplemented with 5000 FTU/kg phytase. Phosphorus solubility increased (P < 0.05) with increasing phytase level. Both methods also identified that mineral solubility plateaus in the