The positive effects of super-dosing phytase on the growth performance of growing pigs are mainly due to increased dietary digestible phosphorus.

This study investigated the effects of super-dosing phytase (up to 3,000 phytase units/kg-FTU/kg) and supplementary dietary inositol (2 g/kg) on phosphorus (P) digestibility and retention (experiment 1), and the growth performance (experiment 2) of growing pigs fed either P-deficient or P-adequate diets. In experiment 1, a total of 96 Duroc × Landrace × Yorkshire barrows [initial body weight (BW) = 37.2 ± 0.4 kg] were housed individually in 32 stainless steel metabolism cages. The barrows were blocked by BW and randomly allocated to 8 treatment groups, using a randomized complete block design. Experiment 1 was conducted in 3 runs to complete 12 replicate pigs per group. Total feces and urine were collected to determine the digestibility and retention of P. In experiment 2, a total of 2,496 Duroc × Landrace × Yorkshire pigs (initial BW = 24.6 ± 0.3 kg) were blocked by BW and sex and randomly allocated to 8 treatment groups, using a randomized complete block design with 12 replicates of 26 pigs per pen, per group. The treatment setup was the same across both experiments. The 8 experimental treatments comprised 2 total P levels (deficient, total P = 0.45% and adequate, total P = 0.60%). Within each P level, 4 diets were fed: control (without added phytase), control plus inositol (2 g/kg), control plus phytase (500 FTU/kg), and control plus phytase (3,000 FTU/kg). Phytase supplementation significantly enhanced P digestibility and retention across both P levels. Phytase supplementation at 500 and 3,000 FTU/kg improved P digestibility from 50.3% to 74.4% and 81.3%, respectively, in P-deficient diets, and from 59.2% to 73.5% and 78.0%, respectively, in P-adequate diets. Similarly, P retention increased from 36.6% to 62.6% and 69.6% in P-deficient diets and from 46.0% to 61.6% and 66.3% in P-adequate diets. In addition, phytase supplementation significantly improved the growth performance of pigs, resulting in increased final BW, average daily gain, and gain-to-feed ratio over the 42 d experimental period. Supplementary dietary inositol alone had no observable impact on P metabolism or growth performance. The interaction between dietary P level and phytase supplementation suggested that the optimal benefit of phytase supplementation may depend on the baseline dietary P content. These results suggested that phytase super-dosing enhanced P digestibility and utilization, reduced P excretion, and optimized growth performance in P-deficient diets. These benefits were mainly driven by the digestible P released by phytase, rather than the presence of inositol.