Effect of dietary L-serine supplementation on serum immunoglobulins of laying hens fed the low crude protein diet. A, B and C respectively show the serum immunoglobulin A, immunoglobulin G and immunoglobulin M contents of layers. Control, the control diet (16.49% CP); LCP, the low crude protein diet (14.05% CP); LCP + Ser, the low crude protein diet supplemented with 0.498% L-serine. Data are represented with the means ± SEM (n = 8). Bars with no common small letters (a, b) indicate statistical differences among 3 treatments (P < 0.05).