Abstract
This study was conducted to evaluate the effects of supplemental L-threonine and enzyme in a wheat-based starter diet (WBD) on the growth performance, intestinal morphology, and immune responses of male ROSS 308 broiler chickens. A total of 240 one-day-old chicks were allocated to six different dietary treatments in a completely randomized design with a 2X3 factorial arrangement. The treatments were two WBD (with or without enzyme) each supplemented with three levels of L-threonine (100%, 120%, and 140% of the NRC recommendation). To evaluate the humoral immune system, sheep red blood cells (SRBC) antigen was injected, and for evaluation of cutaneous basophil hypersensitivity (CBH) response, the phytohemagglutinin-P was injected subcutaneously. On day 21, two chicks were selected out of each replicate and were bled, slaughtered, and then their internal organs were dissected and weighed. Blood samples were used for malondialdehyde (MDA) measurement. Jejunal and ileal contents and tissues were sampled for the viscosity and morphometric measurements. The supplemental L-threonine at the level of 120% of the NRC recommendation improved body weight gain, feed conversion ratio, and anti-SRBC titer (P < 0.05). Also, the supplemental L-threonine at the level of 140% of the NRC recommendation increased the jejunal crypt depth and decreased the jejunal and ileal crypt width (P < 0.05). Nevertheless, the supplemental L-threonine did not affect serum MDA, the viscosity of intestinal contents, and the CBH response (P > 0.05). The enzyme supplementation decreased the viscosity of jejunal contents and increased the villus height to crypt depth ratio in the jejunum (P < 0.05). In conclusion, it seems that more dietary L-threonine is needed to support the performance and health of the broilers when fed the WBD. However, no synergism was observed when L-threonine and enzyme supplementation were simultaneously applied in this study.