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

Ahasic, E., P. Utterback and C. Parsons

Two experiments were conducted to determine P digestibility or bioavailability in dehulled solvent-extracted soybean meals (SBM) for 4 varying particle sizes. Experiment 1 was a CRD 4 x 2 factorial treatment arrangement of 4 mean SBM particle sizes and 2 dietary Ca levels with 6 replicate pens of 5 chicks for each dietary treatment. Experiment 2 was a CRD with 11 dietary treatments and 5 replicate pens of 4 chicks for each dietary treatment. In Experiment 1, Ross 308 chicks were fed 1 of 8 diets from 16-20 d of age. The diets were formulated to contain 0.2% or 0.75% Ca and 4 SBM with mean particle size of 466, 809, 1,174, or 1,577 μm. Ileal digesta and excreta were collected and analyzed for P. In Experiment 2, crossbred chicks (New Hampshire x Columbian) were fed 1 of 11 experimental diets from 8-22 d of age. The basal diet was a P- deficient-cornstarch-dextrose-SBM diet (0.14% nonphytate P). The remaining diets were the basal diet supplemented with 0.05 or 0.10% P from KH2PO4 or 12.5 or 25% of 1 of the 4 above SBM. The right tibia was collected from each euthanized chick for bone ash analysis. Data for both experiments were analyzed using the GLM procedure in SAS. Differences among treatments were considered significant at P<0.05. For Experiment 1, a 4 x 2 factorial analysis was used with 4 SBM particle sizes and 2 dietary Ca levels. For Experiment 2, the slope-ratio method was used to determine the bioavailability of the P in the 4 SBM relative to the P in KH2PO4 using the regression coefficients obtained from multiple regression of tibia bone ash regressed on supplemental P intake from KH2PO4 or the 4 SBM. Bioavailability values of the P in the SBM were calculated relative to KH2PO4 with bioavailability of P in KH2PO4 set at 100%.Ileal P digestibility was not affected by SBM particle size when dietary Ca was 0.2%. At 0.75% dietary Ca, the 1,577 μm SBM yielded the highest ileal P digestibility (65%) and was different (P<0.05) from the other SBM (53%). Chicks fed diets containing 0.2% Ca had higher (P<0.05) ileal P digestibility (88%) and excreta P retention (57%) compared with chicks fed diets containing 0.75% Ca (56% and 42%, respectively). In Experiment 2, the 1,577 μm SBM yielded the highest bioavailability (30%) which was higher (P<0.05) than that for 466 μm SBM (20%).

The results of this study indicated that increased particle size may increase bioavailability of P in SBM and that dietary Ca level greatly affected SBM P digestibility values.