Comparison of values for standardized total tract digestibility and relative bioavailability of phosphorus in dicalcium phosphate and distillers dried grains with solubles fed to growing pigs.

Two experiments were conducted to compare values for the standardized total tract digestibility (STTD) and the relative bioavailability of P in dicalcium phosphate (DCP) and distillers dried grains with solubles (DDGS) when fed to growing pigs. In Exp. 1, the apparent total tract digestibility (ATTD), the basal endogenous P loss (EPL), and the STTD of P in DCP and DDGS were determined. Eighteen pigs (initial BW: 34.93±1.04 kg) were allotted to 3 cornstarch-based diets in a randomized complete block design and housed individually in metabolism cages. Two diets contained DCP and DDGS, respectively, as the sole source of P and the last diet was a P-free diet that was used to measure EPL from the pigs. Results indicated that the ATTD of P in DCP and DDGS were 86.1 and 58.8%, respectively, and the STTD of P in DCP and DDGS were 93.1 and 63.1%, respectively. The EPL was determined at 174 mg/kg DMI. In Exp. 2, 42 pigs (initial BW: 29.02±2.03 kg) were allotted to 7 dietary treatments in a randomized complete block design. Pigs were housed individually and allowed ad libitum access to feed and water. A basal diet (0.22% P) based on corn, casein, cornstarch, and potato protein concentrate was formulated. Three additional diets were formulated by adding 0.04, 0.08, or 0.12% P from DCP to the basal diet to create diets containing 0.26, 0.30, or 0.34% P. The last 3 diets were formulated by adding 0.04, 0.08, or 0.12% P from DDGS to the basal diet at the expense of cornstarch. Pigs were fed experimental diets for 28 d. They were then euthanized and the third and fourth metacarpals from the right front foot were collected. Metacarpal bone ash and bone P were regressed against P intake for each ingredient and via slope ratio methodology, it was determined that the bioavailability of P in DDGS was 87% relative to that in DCP. It was concluded from this work that the value for relative bioavailability of P in DDGS overestimates the digestibility of P in DDGS and values for the STTD of P, therefore, can not be accurately calculated from values for the relative bioavailability of P. As a consequence, it is necessary to determine the STTD of P in feed ingredients included in diets fed to pigs.