Dietary manipulation of farm animals and poultry with organic sources including medicinally-important plants and their bioactive compounds are considered a prime choice to improve productivity and quality characteristics of meat.These plants and plant-source compounds are widely accepted by the consumers due to their natural origin. In the last decade, a growing interest has been observed for the use of supplementation of these plants and their compounds to improve production performance and health; and chemical, sensorial, nutritional and technological properties of meat, without any detrimental effects to the products and environment.However, the practical application of supplementation of whole plants is often limited because of flavour consideration of meat and random effectiveness due to immense variation in the active components of these plants growing in different seasons, eultivars, varieties, etc.Therefore, supplementation of bioactive ingredients (phytochemicals) from these plants in purified form is known to be important for research and their ultimate use on commercial basis. These phytochemicals are also identified for their mutual effects upon combination with each others.Among phytochemicals, flavonoids are known to be prominent due to their important health benefits and meat quality improvement effects.Nevertheless, studies are scarce that have explored their multidimensional effects from farm performance to quality characteristics of meat.Keeping in view the entire facts, present in vivo study was conducted to observe the multidimensional effects of two flavonoids (genistein and hesperidin) individually and in combination for health and meat quality effects in commercial broiler chickens.In order to validate the synergistic antioxidant effects, an in vitro study was also conducted that further explored the flavonoid-flavonoid interactions in terms of their antioxidant capacity in the presence or absence of gut microbiota.　　 IN VIVO STUDY:　　 The health effects of dietary genistein and hesperidin individually and in combination (1∶4) were observed in lipopolysaccharide (LPS)-challenged broiler chickens.A total of 720 (1)-d old commercial Arbor Acres broiler chicks were randomly divided into 6 treatments, with 6 replicates of 20 birds each.Chicks were fed a basal diet without any additive (control, CON), supplemented with 5 mg kg-1 feed genistein (G5) and 20 mg kg-1 feed hesperidin (H20), or a mixture of genistein and hesperidin (1∶4) with doses of 5 mg kg-1 feed (GH5), 10 mg kg-1 feed (GH10) and 20 mg kg-1 feed (GH20) for 42 d.On day 16, 18 and 20, half birds from each group were separated and injected intraperitoneally Escherichia coli LPS (250 μg/kg BW) to induce the immunological stimulation.The results showed that dietary genistein and hesperidin could not exert any significant differences in body weight gain, feed intake and feed conversion ratio (FCR) of broiler chickens.However, both compounds improved (P＜0.01) the antioxidant status of broilers by altering the total antioxidant capacity (T-AOC), malondialdehyde (MDA) production and superoxide dismutase (SOD) activity at 21 and 42 d of analysis.Dietary compounds also seemed to modify the cellular, humoral and mucosal immunity by altering the immune organ indices (spleen, thymus and bursa; P＜0.01 or P＜0.05), T lymphocyte transformation assay (P＜0.01), phagocytic activity (P＜0.01), intestinal intraepithelial lymphocytes numbers (P＜0.01), neutrophil adhesion rate (P＜0.01) and anti-NDV and anti-AIV antibody titers (P＜0.05) at 21 and 42 d of age.Parameters of intestinal morphometry like villus length, villus width, crypt depth and villus length/crypt depth ratios were also found to improved (P＜0.01 or P＜0.05) by the supplemental genistein and hesperidin.LPS treatment exerts immunomodulatory effects (P＜0.05) for all parameters of immunity at 21 d, while a few negative effects were observed for some morphometric structures of small intestine.Overall, genistein and hesperidin improved the antioxidant status, cellular, humoral and mucosal immunity and the morphometry of small intestine in dose-dependent manner.Generally, the effects of combined genistein and hesperidin supplementation were found better as compared to individual doses.　　 Broilers without LPS were further used to study the meat quality effects.During analyzing the meat quality effects at 42 d, it was observed that dietary genistein and hesperidin improved (P＜0.01) the water holding capacity (WHC) of all treated groups as compared to control group.While, the meat color (L* score) and pH were raised (P＜0.05) for GH20 group.No significant differences in sensory score of breast meat were observed between control and treated groups.Microbial load of spoilage organisms including total viable count, psychrotrophs, lactic acid bacteria, enterobactefiaceae, and moulds and yeast count; and lipid oxidation of breast meat was dropped significantly (P＜0.01) at 0 and 15 d of storage in a dose-dependent manner for all supplemented groups. The proportions of total polyunsaturated fatty acids (PUFA); and ratios of n-6/n-3 fatty acids and polyunsaturated fatty acids to saturated fatty acids (PUFA:SFA) in breast muscles were significantly improved (P＜0.05) by increasing levels of both flavonoids. Slight alteration (P＜0.05) was recorded in the protein and fat contents of breast meat. Cholesterol (CHO) contents of breast muscle (P＜0.01) and liver (P＜0.01); and triglyceride (TG) levels of breast muscle (P＜0.05) were reduced significantly.Whereas, the serum levels of CHO, TG and low density lipoprotein cholesterol (LDLC) were decreased significantly (P＜0.05) with increased levels of high density lipoprotein cholesterol (HDLC).Activity of SOD, T-AOC and MDA contents was also found to improve (P＜0.05) in liver.Furthermore, dietary genistein and hesperidin modulated (P＜0.05) the creatine kinase (CK), lactate dehydrogenase (LDH), T-AOC, SOD and heat shock protein 70 (HSP70) mRNA levels of chicken breast muscle.These results suggest the potential of genistein and hesperidin to modulate the quality of chicken meat in a positive direction with pronounced effects of combinatorial treatments.　　 IN VITRO STUDY:　　 This study was conducted to validate the antioxidant effects of genistein and hesperidin observed in vivo study, particularly their paired effects.Both flavonoids were evaluated individually and in combination (molar ratio 1(∶)1) for their antioxidative capacity, in three different concentrations (low, medium and high), in the presence or absence of chicken cecal microbiota.The antioxidative activity was measured through different assays including total antioxidant capacity through β-carotene-linoleic acid model system, superoxide radical scavenging assay, DPPH assay, hydroxyl radical scavenging assay and reducing power assay.Results revealed that genistein and hesperidin have equivalent or higher antioxidant activities comparing with standard control BHT (butylated hydroxytoluene) or quercetin, regardless of addition of microbiota.Generally, a concentration-dependent raise was recorded in the antioxidant activity of individual and paired flavonoids.In some assays, cecal microbiota produce a slight alteration in the outcomes of antioxidant capacity, however this was variable among assays.By calculating combination index, it was known that both genistein and hesperidin could form synergisms; however this tendency was observed comparatively higher in high doses.An immense variation was recorded among the results of all five assays, however, the high concentration of hesperidin was found prooxidant in all assays. Furthermore, a slight reduction was observed in antioxidant activity of flavonoids during incubation of ≥2 h, but this was not noticeable because this was also remained variable among different assays.　　 CONCLUSION:　　 From the outcomes of this study, it could be concluded that genistein and hesperidin have a strong potential to improve health and meat quality of broiler chickens.Combinatorial treatment of both compounds is superior to their individual treatments.Both genistein and hesperidin in combined form (1(∶)4) in the dosage of 20 mg/kg feed could be used on commercial scale to improve the health status and meat quality of broilers.Furthermore, our in vitro results have validated the synergistic effects of genistein +hesperidin, which may assist the further explaining of structure-related activities of plant flavonoids that results their synergistic or antagonistic effects.These results could be helpful in the future research of flavonoids against oxidant-mediated diseases and the paired use of genistein and hesperidin for fortification of foods and preparation of food supplements.