Effect of dietary phosphorus level on the determination of standardized and true total tract digestibility of phosphorus for growing pigs

• There was no effect of dietary P level on apparent total tract digestibility of P.
• The standardized total tract digestibility of P decreased with increasing P level.
• The true total tract digestibility (TTTD) of P was not affected by dietary P.
• The regression method determined values of TTTD of P have relative large variations.

The current study was conducted to investigate the effect of dietary phosphorus (P) concentration on the estimation of standardized total tract digestibility (STTD) and true total tract digestibility (TTTD) of P for growing pigs fed corn- and soybean meal (SBM)-based diets. Fifty-six barrows with an average initial BW of 30.9 kg were used in a randomized complete block design with 7 diets. The 7 diets included a P-free diet and 6 corn- and SBM-based diets that contained the assay ingredient of 150, 300, 450, 600, 750, and 900 g/kg. The assay ingredient consisted of corn and SBM at a fixed ratio of 3:2 by weight. The P-free diet was used to estimate endogenous P loss (EPL) of pigs and to calculate STTD of P in the assay ingredient. The total collection procedure was used to calculate P digestibility with a 5-days adjustment period followed by a 7-days collection period using chromic oxide as a marker to indicate the initiation and termination of fecal collection. The results showed that daily P intake, fecal P output, digested P, and retained P increased linearly with increasing dietary P level for pigs fed corn- and SBM-based diets (P < 0.01). Apparent total tract digestibility (ATTD) of P, urinary P output, and P retention were not affected by dietary P content (P > 0.05). There was a linear decrease in STTD of P (from 54.13 to 43.42%) with graded inclusion of dietary P (P < 0.01). Regressing daily digested P against daily P intake, the TTTD of P were estimated at 40.24 and 38.22% for pigs fed low P diets (the assay ingredient level at 150, 300, and 450 g/kg) and high P diets (the assay ingredient level at 600, 750, and 900 g/kg), respectively. The estimates of TTTD of P were not statistically different between pigs fed low P diets and high P diets (P > 0.05). In conclusion, increasing dietary P from graded inclusion of the assay ingredient reduced the estimated values of STTD of P but did not affect the estimates of TTTD of P determined by the regression method.