2.1. Birds, Experimental Design, Diet, and Management

A total of 640 one-day-old male healthy Arbor Acres broiler chicks with an average weight of 43.06 ± 0.19 g were randomly allocated to 4 experimental groups. Each group had 8 replicates, and each replicate contained 20 birds. Birds in the 4 experimental groups were fed a basal diet with TCE at 0, 30, 60, or 120 mg/kg. The TCE was a 1:1 eutectic of thymol and carvacrol, which was made and provided by Shanghai Institute of Materia Medica, Chinese Academy of Science, China. The composition and nutrient levels of the basal diet are shown in Table 1. The diet was offered in pelleted form. All broilers were housed in cages (length 100 × width 80 × height 60 cm) in a humidity- and temperature-controlled room and had free access to water and feed. At 1–3 days of age, the initial temperature of broilers pens was maintained at 32–35 °C and then gradually decreased until reaching 22 °C, and relative humidity was maintained at 60–70% during 1–10 days of age and then gradually decreased to about 55%.

The composition and nutrition levels of the basal diet (air-dry basis).

⁽¹⁾ Vitamin premix provided the following per kilogram of final diets: vitamin A 7200 IU; vitamin D₃ 1440 IU; vitamin E 60 IU; vitamin K₃ 2.88 mg; vitamin B₁ 1.20 mg; vitamin B₂ 4.32 mg; vitamin B₆ 2.16 mg; vitamin B₁₂ 0.02 mg; biotin 0.29 mg; folic acid 2.40 mg; nicotinamide 24.00 mg; pantothenic acid 15.00 mg. ⁽²⁾ Mineral premix provided the following per kilogram of final diets: copper (CuSO₄·5H₂O) 10.0 mg; iron (FeSO₄·H₂O) 80.00 mg; zinc (ZnSO₄·H₂O) 60.00 mg; manganese (MnSO₄·H₂O) 100.00 mg; iodine (KI) 0.45mg; selenium (Na₂SeO₃) 0.30 mg.