Dynamics of serum phosphorus, calcium, and hormones during egg laying cycle in Hy-Line Brown laying hens.

Understanding serum dynamic patterns of P, Ca, and P-Ca metabolism related hormones would help us in developing feeding strategies that can be used to reduce dietary inorganic P input and decrease P excretion in laying hens. In the current study, Hy-Line Brown laying hens (35-wk-old, n = 15) were fed with a corn-soybean meal-based commercial laying hen diet containing 0.24% non-phytate P, 3.59% Ca, 2,040 IU/kg vitamin D3, 2,500 FTU/kg phytase, 2,636 kcal/kg ME, and 15.9% crude protein. Blood samples from each laying hen were collected immediately after the first oviposition; 3, 6, 9, 12, 15, 18, and 21 h after the first oviposition; and immediately after the second oviposition. As a result, after the first oviposition, serum P and Ca levels of the laying hens were gradually increased, peaked in 6 h, and then gradually decreased all the way down until the second oviposition. Similarly, serum fibroblast growth factor 23 (FGF-23) level was gradually increased after the first oviposition, but peaked in 9 h, and then gradually decreased all the way down until the second oviposition. Serum 1,25-dihydroxy-cholecalciferol [1,25(OD)2D3] level was linearly and quadratically decreased during the egg laying cycle we observed. The serum levels of parathyroid hormone, calcitonin, and alkaline phosphatase were erratically fluctuated during the egg laying cycle in patterns that are very different from that of serum P, Ca, FGF-23, and 1,25(OD)2D3. In conclusion, the result that serum FGF-23 peaked after serum P indicates that the dynamics in serum FGF-23 levels might be driven by serum P levels during the egg laying cycle.