The growth performance, plasma biochemistry indices, immune system, antioxidant status, and intestinal morphology of heat-stressed broiler chickens fed grape (Vitis vinifera) pomace

•GP cause an increase in antioxidant balance in broilers reared under heat stress•GP improve the morphology of jejunal in heat stressed broilers•GP increase the humoral immune response of broilers reared under heat stress This study was conducted to investigate the effects of grape (Vitis vinifera) pomace, GP, on growth performance, blood metabolites, immunity, antioxidant status, and intestinal morphology of heat-stressed broiler chickens. A total of 250 1-d-old broiler chickens were randomly assigned to five treatment groups, with 5 replicates (n = 10). A group was raised at comfort temperatures (thermoneutral) and fed basal diet throughout the study. The other four groups were raised at comfort temperatures until d 24 and then subjected to a heat stress (HS), 37 ± 1 °C and 55% relative humidity, for 6 h from d 25 to 42. They fed diets containing 0, 20, 40, and 60 g GP / kg diet for 42 days. The results showed that GP inclusion linearly increased the feed intake during the starter and grower periods (P = 0.044 and P = 0.023, respectively). Furthermore, dietary GP linearly reduced the blood concentration of triglycerides (P = 0.047), cholesterol (P = 0.007), low-density lipoprotein cholesterol (LDL, P = 0.001), and enzyme activity of aspartate aminotransferase (AST, P = 0.002), but increased the blood concentration high-density lipoprotein cholesterol (P = 0.001) and total protein (P = 0.014) concentrations at d 24. Heat stress impaired the growth performance of broiler chickens. Heat-stressed broiler chickens showed lower carcass efficiency (P = 0.031), breast (P = 0.033), thigh and drumstick (P = 0.003), abdominal fat (P = 0.001), bursa of Fabricius (P = 0.001), and thymus (P = 0.0001) relative weight. The blood protein concentration (P = 0.006), and antibody titer against sheep red blood cells (SRBC, P = 0.002) were lower in heat stressed-birds compared with the thermoneutral broiler chickens. It also decreased the erythrocyte activity of glutathione peroxidase (GPx, P = 0.047) and superoxide dismutase (SOD, P = 0.004), jejunal villus height (P = 0.011), and villus height to crypt depth ratio (P = 0.008). However, HS increased the relative weights of liver (P = 0.001), pancreas (P = 0.001), gallbladder (P = 0.008), and heart (P = 0.022). The HS also increased blood activity of AST (P = 0.003) and concentration of malondialdehyde (MDA, P = 0.02) as compared to thermoneutral condition. Dietary GP did not affect the growth performance, antibody titer agai