Apparent digestibility of energy and nutrients and efficiency of microbial phytase is influenced by body weight of pigs

An experiment was conducted to test the hypothesis that regardless of pig body weight (BW), increasing dietary phytase results in increased phytate degradation and improved digestibility of minerals, amino acids (AA), and gross energy (GE). Eighteen pigs were equipped with a T-cannula in the distal ileum and allotted to a triplicated 6 × 3 Youden square design with six diets and three collection periods of 7 d, for a total of nine replicate pigs per diet. This design was repeated four times to simulate four production phases, and there was a 7-d resting period before each collection phase started (BW at start of collections: 29.3, 53.6, 85.1, and 114.4 kg for phases 1, 2, 3, and 4, respectively). Six corn-soybean meal diets were formulated by including 0, 250, 500, 1,000, 2,000, or 4,000 phytase units/kg feed (FTU). The six diets were used throughout the experiment. Samples of feces and ileal digesta were collected in each period. Results indicated that regardless of pig BW, increasing inclusion of phytase increased (quadratic; P < 0.05) apparent ileal digestibility (AID) of crude protein (CP) and most AA, increased apparent total tract digestibility (ATTD) of Ca, P, K, Mg (linear and quadratic; P < 0.05), and Na (linear; P < 0.05), but decreased (linear and quadratic; P < 0.05) AID and ATTD of GE. In all phases, ileal concentrations of inositol phosphate (IP) 6, IP5, IP4, and IP3 decreased (linear and quadratic; P < 0.05), whereas ileal inositol increased (linear and quadratic; P < 0.05) with increasing dietary phytase. However, as pig BW increased, AID of GE, CP, and AA increased (linear, P < 0.05), and the AID of a few AA (Met, Phe, Thr, Trp, Ala, Asp, Gly, and Ser) also increased quadratically (P < 0.05). The ATTD of GE, K, and Mg increased (linear and quadratic; P < 0.05), but ATTD of Ca and Na (linear; P < 0.05) and of P (linear and quadratic; P < 0.05) decreased as pig BW increased. Ileal IP6 and IP3 (linear and quadratic; P < 0.05) and ileal IP5 and IP4 (linear; P < 0.05) increased, whereas ileal inositol decreased (linear; P < 0.05) as pig BW increased. In conclusion, regardless of pig BW, increasing dietary phytase increased phytate degradation and inositol release in the small intestine, and consequently increased mineral and AA digestibility. Older pigs have reduced Ca, P, and Na digestibility, but increased K, Mg, AA, and GE digestibility compared with younger pigs. The efficiency of dietary phytase to degrade phytate appears to decrease as pigs get older.