Abstract: [Objective] To investigate the effects of dietary supplementation with different levels of red yeast rice powder on the growth performance, immune performance, and serum biochemical indices of yellow-feathered broilers, and to provide a reference for its rational application in broiler production. [Methods] A total of 240 one-day-old slow-growing yellow-feathered broilers were randomly divided into 4 groups, with 3 replicates per group and 20 birds per replicate. The control group was fed a basal diet, while the experimental groups were fed the basal diet supplemented with 2%, 4% and 6% red yeast rice powder （designated as the 2% group, 4% group and 6% group, respectively）. All experimental yellow-feathered broilers were vaccinated with Newcastle disease-infectious bronchitis-avian influenza （H9） triple inactivated vaccine at 10 days of age. The experimental period lasted 42 days. During the experiment, body weight and feed intake of the broilers were regularly measured, and average daily gain, average daily feed intake and feed-to-gain ratio at each stage were calculated. Serum samples were collected at 21 d and 42 d to determine Newcastle disease （ND） and avian influenza H9 subtype （AI H9） antibody titers as well as related serum biochemical indices. At 21 d and 42 d, three chickens were randomly selected from each group, weighed and sacrificed, and the liver, stomach, intestine, bursa of Fabricius, thymus and spleen were separated to calculate organ indices. [Results] From 1 to 21 d, there were no significant differences in growth performance indices among the groups （P>0.05）. From 22 to 42 d, the average daily gain of all experimental groups was lower than that of the control group, and only the 6% group showed a significant difference compared with the control group （P<0.05）; there were no significant differences in average daily gain between the 2% group or 4% group and all other groups （P>0.05）. The average daily feed intake of the 2% group and 6% group was significantly higher than that of the control group （P<0.05） and extremely significantly higher than that of the 4% group （P<0.01）, while the average daily feed intake of the 4% group was significantly lower than that of the control group （P<0.05）. The feed-to-gain ratio of all experimental groups was higher than that of the control group; the feed-to-gain ratio of the 6% group was significantly higher than that of the control group and the 4% group （P<0.05）, and there were no significant differences among the 2% group, control group and 4% group （P>0.05）. Throughout the entire experimental period （1-42 d）, there were no significant differences in average daily gain or feed-to-gain ratio between the experimental groups and the control group （P>0.05）; the average daily feed intake of the 4% group was significantly lower than that of the 2% group and 6% group （P<0.05）, and there were no significant differences among the control group, 2% group and 6% group （P>0.05）. At 21 d and 42 d, there were no significant differences in digestive organ indices or immune organ indices among the groups （P>0.05）; there were no significant differences in avian influenza H9 subtype antibody （AI H9 Ab） titers or Newcastle disease antibody （ND Ab） titers among the groups （P>0.05）. At 21 d, the serum total protein （TP） of the three experimental groups was extremely significantly higher than that of the control group （P<0.01）; the serum globulin （GLB） content of the 2% group and 4% group was extremely significantly higher than that of the control group （P<0.01）, and the GLB content of the 6% group was significantly higher than that of the control group （P<0.05）. The alanine aminotransferase （ALT） activity of all experimental groups was lower than that of the control group; the ALT activity of the 6% group was extremely significantly lower than that of the control group （P<0.01） and significantly lower than that of the 4% group （P<0.05）, while the ALT activity of the 2% group showed no significant difference with any group （P>0.05）. There was no significant difference in alkaline phosphatase （ALP） activity between any experimental group and the control group （P>0.05）, but the ALP activity of the 6% group was significantly lower than that of the 2% group and 4% group （P<0.05）. The total cholesterol （T-CHO） content of all experimental groups was lower than that of the control group, among which the T-CHO content of the 4% group was significantly lower than that of the control group （P<0.05）, and there were no significant differences in T-CHO content among the control group, 2% group and 6% group （P>0.05）. The low-density lipoprotein cholesterol （LDL-C） content of all experimental groups was lower than that of the control group; that of the 2% group was extremely significantly lower than that of the control group （P<0.01）, that of the 4% group was significantly lower than that of the control group （P<0.05）, and that of the 6% group showed no significant difference with any group （P>0.05）. At 42 d, the ALT activity of the 6% group was extremely significantly higher than that of the control group （P<0.01）, that of the 4% group was significantly higher than that of the control group （P<0.05）, and that of the 2% group also showed a certain increase compared with the control group （P>0.05）; there were no significant differences in ALT activity among the experimental groups （P>0.05）. The ALP activity of the 2% group was extremely significantly higher than that of the control group （P<0.01） and significantly higher than that of the 4% group （P<0.05）. There were no significant differences in malondialdehyde （MDA） content among the experimental groups （P>0.05）, and the MDA content of all three groups was extremely significantly lower than that of the control group （P<0.01）. The T-CHO content of the three experimental groups was lower than that of the control group, among which the T-CHO content of the 2% group was significantly lower than that of the control group （P<0.05）, that of the 4% group was extremely significantly lower than that of the control group （P<0.01）, and that of the 6% group showed no significant difference with any group （P>0.05）. [Conclusion] Dietary supplementation with 2%-6% red yeast rice powder shows no obvious promoting effect on the growth performance and immune performance of yellow-feathered broilers from 1 to 42 d. Long-term high-dose supplementation of red yeast rice powder has potential risks of inhibiting broiler growth and increasing hepatic metabolic pressure. Supplementation with 2% and 4% red yeast rice powder can reduce blood lipids and improve antioxidant capacity. Comprehensively considered, it is recommended that the addition level of red yeast rice powder in the diet of yellow-feathered broilers from 1 to 42 d should not exceed 2%; when utilizing its antioxidant and lipid-lowering functions, the addition dosage should be strictly controlled or a staged feeding strategy should be adopted, and liver function monitoring should be strengthened.

Key words: red yeast rice powder, broiler, growth performance, immune performance, biochemical indices