Interactive effects of methionine source and carnitine supplementation on growth performance, immunity, antioxidant status, and HSP70 gene expression in broilers reared under heat stress conditions

The current study was aimed at elucidating the effects of methionine source and L-carnitine (CT) on performance, immunity, stress indicators, and antioxidant status in broilers kept under chronic heat stress (HS). A total of 720 one-day-old (Ross 308) male broiler chickens were assigned to 5 dietary treatments with 6 replicates and 24 chickens per replicate. The 4 HS treatments included 2 sources of methionine (DL-methionine vs. the DL-methionine-hydroxy analogue [MHA]) and 2 levels of CT supplementation (0 and 200 mg CT/kg of diet). A thermoneutral group containing DL-methionine supplement but without CT supplement was considered the experimental control group (TN-CON). The HS groups were kept in a cyclic HS environment at 33°C to 34°C for 8 h (10:00–18:00) per day beginning on d 3. Dietary supplementation with either CT or MHA significantly reversed the HS-induced decreases in uniformity rate and European performance index, as well as the HS-induced increase in the heterophile to lymphocyte ratio. The relative weight of the thymus and bursa, the total antibody titer against various antigens for both primary and secondary responses, the total antioxidant capacity (TAC), glutathione peroxidase activity, concentrations of insulin-like growth factor 1 and malondialdehyde (MDA), and HSP70 expression in the liver and hypothalamus all improved when MHA was substituted for DL-methionine in the diet. Broilers in CT-supplemented groups had lower relative liver weight, fat content in the abdomen, breast, and thigh, plasma MDA and corticosterone concentrations, but higher relative thymus weight, plasma TAC levels, and catalase activity. Broilers fed diets containing both MHA and CT had growth rates, feed:gain ratios, and mortality rates similar to the TN-CON group, despite the lack of interaction effects between methionine source and CT supplementation on any measured parameters. Overall, adding either CT or MHA to the diet of broiler chickens raised in HS conditions improved their antioxidant status, but MHA had a more pronounced effect on enhancing growth performance and stress indicators.