Independent effects of phytase and β-mannanase on growth performance and nutrient utilization in broilers fed phosphorus-deficient diets without supplemental inorganic phosphorus

Phytase and β-mannanase are widely used in poultry diets to improve nutrient utilization; however, their individual and interactive effects under phosphorus (P) restriction without supplemental inorganic P remain insufficiently characterized. This study evaluated the effects of phytase and β-mannanase, alone and in combination, on growth performance, ileal digestible energy (IDE), and apparent ileal calcium (Ca) and P digestibility in broilers fed a P-deficient diet without supplemental inorganic P.

A total of 720 one-day-old Ross 308 male broilers were randomly allocated to four dietary treatments in a 2 × 2 factorial arrangement comprising phytase at 0 or 1,000 FTU/kg and β-mannanase at 0 or 800 Thermostable mannanase unit/kg. Each treatment included 12 replicate pens of 15 birds. From days 8 to 24, birds received corn-soybean meal-based grower diets containing 0.40% total P, 0.14% non-phytate P, and 0.26% phytate P without supplemental inorganic P. Growth performance was evaluated, and ileal digesta were collected on day 24 to determine IDE and apparent ileal Ca and P digestibility.

Phytase supplementation increased final body weight by 14.4%, average daily gain by 17.8%, and feed intake by 14.4% and improved feed conversion ratio by 2.9% (p < 0.01). Phytase also increased IDE from 2,641.84 to 2,686.30 kcal/kg, Ca digestibility from 57.82% to 58.83%, and P digestibility from 71.59% to 74.83% (p < 0.01). β-Mannanase increased IDE from 2,647.01 to 2,681.13 kcal/kg (p < 0.01) and Ca digestibility from 58.00% to 58.70% (p < 0.05) but did not significantly affect growth performance or P digestibility. No significant phytase × β-mannanase interactions were detected for growth performance, IDE, or mineral digestibility, although an interaction was observed for flock uniformity (p = 0.011).

Under the P-restricted dietary conditions, phytase primarily improved growth performance and mineral utilization, whereas β-mannanase independently enhanced energy utilization and modestly improved Ca digestibility. The absence of synergistic effects indicates distinct enzyme responses under these dietary conditions. These findings support phytase as a nutritional strategy for substantially reducing supplemental inorganic P in carefully formulated broiler diets, although longer-term validation under commercial feeding conditions is warranted.