This study investigated the effects of methionine source and L-carnitine (CT) on performance, immunity, stress indicators, and antioxidant status in broilers under chronic heat stress. A total of 720 one-day-old male broiler chickens were divided into 5 dietary treatments with 6 replicates and 24 chickens per replicate. The 4 HS treatments included 2 sources of methionine [DL-methionine and DL-methionine-hydroxy analogue (MHA)] and two levels of CT supplementation (0 and 200 mg/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 33-34°C for 8 h (10:00-18:00) per day beginning on day 3. Supplementing with CT or MHA reversed the negative effects of HS on uniformity rate, European performance index, and the heterophile to lymphocyte ratio. Replacing DL-methionine with MHA in the diet improved relative weights of the thymus and bursa, total antibody titer against various antigens, total antioxidant capacity (TAC), glutathione peroxidase activity, insulin-like growth factor 1 and malondialdehyde (MDA) concentrations, and HSP70 expression in the liver and hypothalamus. Broilers in CT-supplemented groups also 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. Although there were no interaction effects observed between methionine source and CT supplementation on the measured parameters, broilers fed diets containing both MHA and CT exhibited comparable growth rates, feed:gain ratios, and mortality rates to the TN-CON group. Overall, both CT and MHA improved antioxidant status in heat-stressed broilers, but MHA had a more pronounced effect on enhancing growth performance and stress indicators.
