Effects of chitosan oligosaccharides (COS) on antioxidant status of the small intestine in yellow-feather heat-stressed broilers. CON group, basal diet and raised under normal temperature (24°C); HS group, basal diet and raised under cycle heat stress (34°C from 10:00–18:00 and 24°C for the rest time); HSC group, basal diet with 200 mg/kg COS supplementation and raised under cycle heat stress. Abbreviations: CAT, catalase; GSH-Px, glutathione peroxidase; MDA, malondialdehyde; SOD, superoxide dismutase. Values are mean ± SE (n = 6). The values with different superscript letters are different (P < 0.05).