Determination of available and total tract digestible phosphorus release curves for Microtech phytase

A total of 320 barrows (DNA 200 × 400; initially 10.7 ± 0.21 kg) were used in a 21-d growth trial to determine the available P (aP) and true total tract digestible P (TTTD P) release curves for Microtech phytase (VTR Bio-Tech Co, Ltd, Guangdong, China). At ∼21 d of age, pigs were weaned, randomly allocated to pens and fed a common diet for 18 d and then fed a P deficient diet with 0.11% aP and 0.20% standardized total tract digestible (STTD P) for 3 d. On d 21 post-weaning, considered d 0 of the study, pigs were blocked by average pen body weight (BW) and randomly allotted to 1 of 8 dietary treatments with 5 pigs per pen and 8 pens per treatment. Dietary treatments were derived from a single basal diet and phytase, monocalcium phosphate, limestone, and sand were added to create the treatment diets. The basal diet was formulated to contain 0.32% phytate P. Treatments included 3 diets with 0.11%, 0.19% and 0.27% aP (corresponding to STTD P of 0.20%, 0.27% and 0.35%) using monocalcium phosphate and 5 diets with 0.11% aP (0.20% STTD P) and increasing phytase (250, 500, 1000, 1500 and 2000 FTU/kg). On d 14 of the trial, fecal samples were collected from 3 pigs per pen. Samples were analyzed for dry matter (DM), N, P, and TiO₂ for the calculation of apparent total tract digestibility (ATTD) of DM, N and P. At the conclusion of the experiment, 1 pig, closest to the mean weight of each pen, was euthanized for bone analysis. The right fibula, 10th rib, and third metacarpal were collected for the determination of bone density, bone ash weight, and percentage bone ash. Increasing aP and STTD P from inorganic P increased (quadratic, P ≤ 0.050) final BW, average daily gain (ADG), and gain to feed ratio (G:F). Increasing phytase increased (linear, P ≤ 0.012) final BW and average daily feed intake (ADFI) and improved (quadratic, P ≤ 0.042) ADG and G:F. Dry matter digestibility decreased (quadratic, P = 0.043) with increasing aP and STTD P from inorganic P. Phosphorus digestibility increased (linear, P ≤ 0.001) with increasing aP and STTD P from inorganic P and increasing phytase. Bone density, bone ash weight, and percentage bone ash increased (linear, P ≤ 0.004) with increasing aP and STTD P from inorganic P and increasing phytase. The aP release curve based on the average percentage bone ash of the three bones and TTTD P release curve developed for Microtech phytase are: aP release, % = (0.868 × FTU/kg) ÷ (9599.511 + FTU/kg) and TTTD P release, % = (0.00004017 × FTU) + 0.011, respectively.