Broilers under oxidative stress from high ambient temperatures may benefit from the use of additives that have antioxidant properties. This experiment investigated the efficacy of a herbal extract mixture (HEM; aqueous extracts from Ferula gummosa, Thymus vulgaris, and Trachyspermum copticum) in day-old chicks, injected intramuscular (deep pectoral muscle; (0, 30, 60, and 90 μL/0.1 mL of sterilized and distilled water)), and supplemented in drinking water (0 and 0.25 mL/L) during the rearing period. Broilers were reared in battery cages under summer temperature conditions, with average maximum temperature of 35.5°C, average minimum temperature of 25.5°C, and average relative humidity of 50–60%. A total of 400 1-day-old Ross 308 male broiler chicks were randomly assigned to 8 treatment groups (5 replicates/treatment with 10 birds per replicate). From d1 to d10, the indoor air temperature was adjusted to match fluctuating outdoor summer temperatures, and was set at 30–34°C and 50–60% relative humidity; and from d10 onwards, no adjustments were made. Injection of HEM linearly decreased feed:gain (P = 0.005), heterophile-to-lymphocyte (H/L) ratio (P = 0.007), and serum concentrations of cholesterol (P = 0.008), low-density lipoprotein cholesterol (LDL) (P < 0.001), malondialdehyde (P = 0.005), and cortisol (P = 0.008). The 60 μL of HEM injection produced the best results in terms of final body weight (BW; P = 0.003), overall average daily gain (ADG; P = 0.002), European performance index (P < 0.001), carcass yield (P < 0.001), and serum glutathione peroxidase activity (P < 0.001). Supplementation of HEM in drinking water also increased final BW (P = 0.048), overall ADG (P = 0.047), high-density lipoprotein cholesterol (P = 0.042), and total antioxidant capacity (P = 0.030), while decreasing the H/L ratio (P = 0.004) and serum LDL concentration (P = 0.031). There were interactions between injection and water supplementation for BW (day 24; P = 0.045), carcass yield (day 42; P = 0.014), and serum superoxide dismutase activity (day 42; P = 0.004). In conclusion, administering an injection of HEM at a dose of 60 μL at the time of hatching, followed by supplementation at a dose of 0.25 mL/L via drinking water during the rearing period could be a useful strategy for improving the performance and health status of heat-stressed broiler chickens.
