Abstract
Babcock Brown layers (n = 960), 40 wk of age, were allocated to 1 of 5 dietary treatments groups, each of which included 6 replicates of 32 hens. Each group received the same basal diet formulated with corn, peanut meal, and crystalline amino acids for 8 wk. L-Threonine was added to the basal diet at 0 (control), 0.1, 0.2, 0.3, and 0.4%, to achieve 0.47 (NRC, 1994), 0.57, 0.67, 0.77, and 0.87% threonine, respectively. Although supplementing the diet with L-threonine did not affect ADFI, FCR, egg weight, or egg quality (P > 0.05), the egg production response to supplemental L-threonine was quadratic, and it was maximized at 0.2% supplemental L-threonine. No differences were observed for uric acid, lactate dehydrogenase, alkaline phosphatase, Ca, and P concentrations among the treatments. Serum total protein concentration increased quadratically to supplemental threonine, and the response was maximized at 0.2 and 0.3% supplemental L-threonine. Serum free threonine increased quadratically as supplemental threonine increased, and the response was maximized between 0.2 and 0.3% supplemental L-threonine. The addition of L-threonine at 0.3% of the diet resulted in linearly increasing levels of IgG and total Ig (P < 0.05) as compared with those of the control group. In conclusion, current NRC (1994) recommendations for dietary threonine are insufficient for modern commercial laying hens strains reared under subtropical summer climates. We suggest that 0.2% threonine resulted in optimal egg production, whereas 0.3% L-threonine may have had a positive effect on the humoral immune response of laying hens under conditions of high temperature and humidity.