Effect of zinc level and fat source on the performance and immunocompetence of heat-stressed broilers

Two experiments utilizing male broilers were conducted to determine the effect of different levels of dietary zinc, and sources of dietary fatty acids on growth performance and immune competence of broilers raised under heat stress conditions. Broilers were raised in thermoneutral (IN, 23.9 0), or heat stress (HS, 35 C) environments. In experiment 1, broilers were fed low zinc (Low Zn, 32 mg/kg), control ( NRG recommendation), and high Zn (control + 100 mg Zn/kg) diets. In experiment 2, the birds were fed corn oil, fish oil, and fish oil + Zn diets. Cellular immunity was assessed by using a Sephadex stimulation method to recruit abdominal exudate cells (AEG) and their phagocytic ability determined. Humoral immunity was determined by injecting birds intravenously with 1 ml of 7% sheep red blood cells (SRBC) followed by evaluation of serum for total, IgM, and IgG antibody titers.

Results indicated that HS consistently caused a reduction in growth performance as well as in both cellular and humoral immune responses. Zn did not impact growth performance. Lymphoid organ weights, primary and secondary antibody responses, incidence of macrophages in AEG, phagocytic ability of macrophage for opsonized and unopsonized SRBC, and plasma and tibia Zn concentrations were all significantly reduced by heat stress.

Total, IgM, and IgG antibody titers were highest (P <0.001) among birds fed high Zn diets for both primary and secondary responses. Number of AEG recruited (P< 0.01), number of macrophages in AEC (P < 0.008), number of phagocytic macrophages, as well as the number of internalized SRBC for opsonized and unopsonized SRBC (P < 0.001), were all increased with high Zn diets. Plasma Zn concentration was unaffected by level of Zn, while tibia Zn concentration increased (P < 0.001) with high Zn.

Performance of broilers was unaffected by dietary fatty acids. Lymphoid organ weights were not changed with added fat. Total, IgM, and IgG antibody titers were increased (P < 0.001) with fish oil + Zn for both primary and secondary responses. Number of AEC, and macrophages recruited were not affected by dietary fatty acids. Phagocytic ability and number of internalized SRBC were increased (P < 0.002) with both fish oil diets.

Feeding broilers diets supplemented with Zn, as well as fish oil may improve the birds' ability to mount an effective immune response. (Key words: Zinc, heat stress, broilers, fatty acids, immune response).