Immunological Responses of Laying Hens to Dietary Crude Protein and Threonine

An experiment was conducted to study the effects of dietary crude protein (CP) and threonine (Thr) on immunological responses of laying hens. The 48-wk old layers fed 9 experimental diets consisting of 3 levels of dietary CP (90, 95, and 100% of Hy-Line W-36 recommendations) and Thr (100, 110, and 120% of Hy-Line W-36 recommendations) throughout experiment. Statistical analysis showed that dietary CP did not significantly affect the pattern of serum protein, leukocyte count (LC), humoral and mucosal immunity, relative weight of pancreas, liver, and spleen. However, feeding low-CP diets reduced total protein (P=0.17), albumins (P<0.01) and beta-globulins (P<0.05) in the serum. Increasing dietary levels of Thr in low-CP diets increased the proportion of serum gamma globulins, secondary immune response against sheep red blood cell (SRBC) antigen, goblet cell number and relative weight of pancreas (P<0.05). Supplementing low-CP diet with highest level of Thr linearly increased antibody production against SRBC (P<0.05). The results indicated that although decreasing dietary CP had no adverse effects on the immune functions of laying hens, the optimal systemic and mucosal immunity might be achieved by increasing dietary Thr in low-CP diets.