Dietary calcium, non-phytate phosphorus, and electrolyte balance in low-protein diets for laying hens: Evaluating...

The close relationship between nitrogen and macro-mineral nutrition in laying hens is well-established. Given the increasing promotion of low-protein diets, there is now a critical need to reassess the requirements and balance of calcium (Ca), non-phytate phosphorus (NPP), and electrolytes under such dietary conditions. We conducted a 3 × 3 factorial experiment using 756 Hy-Line Brown laying hens (49 weeks old). Birds were fed a 15.0 % crude protein diet and allocated into nine groups (7 replicates of 12 hens each). The two main factors were: (1) Ca and NPP: Ca 3.50 %, NPP 0.32 % (NY/T 33-2004); Ca 3.91 %, NPP 0.38 % (Hy-Line Brown commercial management guide); Ca 4.20 %, NPP 0.30 % (Dutch CVB standards). (2) Dietary electrolyte balance (DEB) levels: 75 mEq/kg was added to or subtracted from the basal diet's DEB level (45, 120, and 195 mEq/kg). The interaction between Ca, NPP, and DEB affected blood creatinine and phosphorus (P) levels, apparent total tract digestibility (ATTD) of dry matter, crude protein, and duodenal mRNA expressions of Na⁺/K⁺-ATPase α1 subunit (ATP1A1) and solute carrier family 2 member A9 (SLC2A9, P < 0.05). Compared to the Ca 3.91 %, NPP 0.38 % group, the Ca 3.50 %, NPP 0.32 % group showed improved eggshell strength (P < 0.001), albumen height (P < 0.001), and Haugh unit (P = 0.018). The elevated blood P and creatinine levels, duodenal ATP1A1 and SLC2A9 mRNA expression observed in the Ca 4.20 %, NPP 0.30 % group may indicate an increased physiological burden on renal function. The DEB 195 mEq/kg group showed enhanced dry matter and crude protein ATTD (P < 0.05), and duodenal renal transient receptor potential vanilloid type 5 (TRPV5) expression compared to the DEB 45 and 120 mEq/kg groups (P = 0.024). Collectively, among the tested levels, the existing NY/T 33-2004 recommendations for Ca and NPP (Ca 3.50 %, NPP 0.32 %) remain effective in low-protein diets when combined with an elevated DEB of 195 mEq/kg.