A comparison of the test tube and the dialysis tubing in vitro methods for estimating the bioavailability of phosphorus in feed ingredients for swine

The validity of a simplified in vitro test tube (TT) method was compared with a more complicated dialysis tubing (DT) method to estimate the percentage of available phosphorus (P) in 41 plant origin and five animal origin feed ingredients for swine. The TT method using 1.0 or 0.25 g samples was compared with the DT method using 1.0 g samples at two pancreatic incubation times (2 vs 4 h) in a 3 x 2 factorial arrangement of treatments. Each DT and TT method treatment was replicated three and six times, respectively. Both methods utilize three enzymatic digestions: (i) predigestion with endoxylanase and beta-glucanase for 1 h, (ii) pepsin digestion for 2 h, and (iii) pancreatin digestion for 2 or 4 h. For the TT method, the entire procedure was conducted in a 50 mL conical centrifuge tube and replicated six times. For the DT method, the first two digestions were conducted in a 10 mL plastic syringe before the contents were quantitatively transferred into a segment of DT for the pancreatic digestion. The percentages of hydrolyzed P for plant origin ingredients measured by the DT method using 1.0 g samples and the TT method using 0.25 g samples were highly correlated (r = 0.94-0.97, P < 0.001) with each other and with published in vivo available P values for swine. Repeatabilities for these two methods ranged from 99.64 to 99.86%. The TT method using 1.0 g samples, however, did not provide valid estimates of P availability for all ingredients. For animal origin ingredients, neither method was significantly correlated (r = 0.1-0.6, P >or = 0.4) with published in vivo available P values. In conclusion, the accuracy and validity of the TT method using 0.25 g samples with a 2 h pancreatic digestion time was equal to or superior to the DT method using 1.0 g samples with a 4 h pancreatic digestion time for estimating P availability in plant origin feed ingredients.