Comparison of two diet types in the estimation of true digestibility of phosphorus in soybean and canola meals for growing pigs by the regression method

- Diet type does not affect the estimation of true P digestibility in assay ingredients for pigs by the regression method.
- Feeding the corn-based practical diet leads the determined values of endogenous P loss (EPL) were negative.
- The P content and digestibility in feedstuffs included in the basal diet affect the determination of EPL.

Two experiments were conducted to compare 2 types of basal diet (semi-purified cornstarch diet and corn-based practical diet) in the determination of true P digestibility (TPD) in soybean (SBM) and canola (CM) meals for growing pigs by the regression method. In each study, forty-eight 25-kg barrows were assigned to 6 dietary treatments arranged in a 2×3 factorial of 2 types of diet (semi-purified cornstarch diet or corn-based practical diet), and 3 graded levels of SBM (Exp. 1) or CM (Exp. 2) at 120, 240, or 360 g/kg. Both experiments consisted of a 5-d adjustment period and a 7-d total collection period with ferric oxide used as a marker to indicate the initiation and termination of fecal collection. Our results showed that dietary P intake, fecal P output, digested P, and retained P were increased linearly with graded increasing levels of SBM or CM for growing pigs in each type of diet (P<0.01). Regression of digested P against dietary P intake resulted in the estimated TPD in SBM for pigs fed the semi-purified cornstarch diet and the corn-based practical diet to be 40.01 and 37.49%, respectively. The respective estimates of TPD in CM were calculated to be 30.19 and 27.22% for pigs fed the semi-purified diet and the practical diet. There was no difference in the estimates of TPD in SBM and CM between pigs fed the semi-purified diet and the practical diet. In summary, our data indicated that diet type (semi-purified or practical diet) does not influence the determination of TPD in assay ingredients for growing pigs by the regression method. Therefore, a semi-purified cornstarch diet can be used for determination of P digestibility.