The present study aimed to investigate the effect of encapsulated Cosmos caudatus leaf extract on the physiological conditions, immune competency, and antioxidative status of broiler chickens raised at a high stocking density. After 15 days of rearing, 370 Lohmann broiler chicks were assigned to five treatment groups, including T0 (chicks were raised at a density of 10 birds/m2 and received no additive), T1 (chicks were raised at a density of 16 birds/m2 and received no additive), T2 (chicks were raised at a density of 16 birds/m2 and received 0.5 g/kg encapsulated C. caudatus leaf extract), T3 (chicks were raised at a density of 16 birds/m2 and received 1.0 g/kg additive), and T4 (chicks were raised at a density of 16 birds/m2 and received 1.5 g/kg additive). On days 28 and 42, blood samples from two chicks per pen were collected. On day 42, the chicks that had been blood-sampled were sacrificed, and blood samples and lymphoid organs (i.e., bursa of Fabricius, spleen, and thymus) were collected. The daily weight gain and feed efficiency of broilers (P<0.01) in groups T2 and T3 were higher than those of broilers in groups T0, T1, and T4. Daily feed intake was greater (P<0.01) in groups T0 and T1 than in groups T2 and T3. The erythrocyte content and hematocrit value of groups T1, T2, T3, and T4 were greater (P<0.05) than those of group T0. The mean corpuscular hemoglobin concentration in group T4 was lower (P<0.05) than that in groups T0, T1, and T2. Leukocyte and lymphocyte levels were higher in group T1 (P<0.05) than in other groups. Serum albumin was higher in chicks reared at a high density (P<0.05) than in chicks reared at a normal density. Lesion scores were higher in group T1 (P<0.05) than in other groups. Chicks in groups T1 and T2 showed more severe pathological changes in their bursa of Fabricius compared with those in groups T0, T3, and T4. Serum superoxide dismutase was higher in groups T2, T3, and T4 (P<0.05) than in groups T0 and T1. Chicks in group T4 had higher (P<0.05) malondialdehyde levels than chicks in other groups. In conclusion, a high stocking density influences the metabolic rate and physiological conditions of broiler chicks, as reflected by alterations in the blood profiles of the animals. Stress due to a high stocking density could damage the bursa of Fabricius, but feeding with encapsulated C. caudatus leaf extract, especially at a rate of 1.5 g/kg, could alleviate the cortical and lymphocyte cell depletion of broilers. Regardless of the stocking density effect, dietary supplementation with encapsulated C. caudatus leaf extract at doses of 0.5 and 1.0 g/kg could improve the daily weight gain of broilers.
