Reconsidering amino acid matrix values: 6-phytase benefits beyond phosphorus release but limited growth recovery in lysine-deficient diets

The efficancy of commercial 6-phytases in reduced digestible lysine (dig. Lys) diets, formulated to the ideal amino acid (AA) profile, remains insufficiently studied. A total of 896 Ross 308 broilers were assigned to seven dietary treatments for 41 days (8 replicates × 16 birds). Diets included a positive control (T1; dig. Lys per Ross 308 guidelines, no phytase), a negative control (T2; 3% dig. Lys below guidelines, no phytase), and five NC diets with different 6-phytases (T3–T7). Available phosphorus (avP) in T1 and T2 was supplied via dicalcium phosphate, while phytase diets were formulated with matrix contributions of 0.19% calcium (Ca), avP, and 0.03% Na. Parameters assessed were performance, intestinal morphology, caecal short-chain fatty acids (SCFAs), nutrient digestibility, tibia mineralization, and microbial counts. T1 birds showed significantly higher (p < 0.05) BW and BWG than all others. Despite partial recovery with phytase, FCR increased (p < 0.05) by 3.85–5.12% in T3–T7 compared to T1. Acetate and total SCFAs increased in phytase groups (notably T4, T5), while butyrate rose in T5 and T7. T5 birds had the tallest villi, and T4–T5 recorded the largest villus surface area. Tibia ash and P were elevated in T5 and T7. Digestibility of phosphorus, crude protein (CP), and dry matter increased with phytase, peaking in T4–T7 (p < 0.05). Aerobic and lactic acid bacteria increased in T4–T7, while coliforms were unaffected. Certain phytases, especially in T4, T5, and T7, improved gut health, nutrient utilization, and bone mineralization in reduced-dig. Lys diets, though growth performance was not fully restored. In conclusion, phytase manufacturers may need to re-evaluate their recommended AA matrix values.