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.