Phosphorus digestibility and relative phosphorus bioavailability for soybean meals varying in particle size

Two experiments were conducted to determine phosphorus (P) digestibility or bioavailability in dehulled solvent-extracted soybean meals (SBM) varying in particle sizes. Experiment 1 was a 4 × 2 factorial treatment arrangement of 4 mean SBM particle sizes (466, 809, 1,174, and 1,577 μm) and 2 dietary calcium (Ca) levels (0.20 and 0.75%) and was conducted with Ross 308 broiler chicks to determine ileal P digestibility and total tract P retention at 20 d of age. Ileal P digestibility values (86 to 90%) were not affected by SBM particle size when dietary Ca was 0.2%. However, at 0.75% dietary Ca, 1,577 μm SBM yielded highest ileal P digestibility and was different (P < 0.05) from other SBM particle sizes. Chicks fed diets containing 0.20% Ca had higher (P < 0.05) ileal P digestibility and total tract P retention values when compared with chicks fed diets containing 0.75% Ca. Experiment 2 was conducted to determine relative P bioavailability in the 4 SBM relative to potassium phosphate (KH₂PO₄) using a tibia bone ash assay. Crossbred chicks (New Hampshire x Columbian) were fed one of 11 experimental diets from 8 to 22 d of age. Diet 1 was a P-deficient cornstarch-dextrose-SBM diet (0.14% nonphytate P). Remaining 10 diets were Diet 1 supplemented with 0.05 or 0.10% P from KH₂PO₄ or 12.5 or 25% SBM that had a mean particle size of 466, 809, 1,174, or 1,577 μm. Bioavailability of SBM P relative to KH₂PO₄ was estimated using the slope ratio method for multiple regression of tibia ash regressed on supplemental P intake. The 1,577 μm SBM yielded highest relative bioavailability values (30-36%) based on bone ash which were higher (P < 0.05) than the values for 466 μm SBM (20-25%). Results of this study indicate that increased particle size may increase digestibility and relative bioavailability of P in SBM and that dietary Ca level greatly affected SBM ileal P digestibility values.