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Simple Summary: Currently, the beef production system faces important challenges, such as improving feed efficiency, reducing environmental impact, and improving animal welfare. Citrus flavonoids from bitter orange plant secondary metabolites are feed additives that have shown promising effects on intake modulation, efficiency, and improving animal behaviors related with animal welfare. However, as they interact with the digestive tract microbiota and the digestive tract receptors, their effects may be affected by the feeding method (mash or pellet). In the present study, when these flavonoids were fed in a pellet concentrate presentation form, the bull's efficiency did not improve. However, animal behaviors related to welfare problems were reduced. Furthermore, supplementing bulls with flavonoids modified the expression in the rumen of genes concerned with behavior and inflammatory response. Therefore, supplementing bulls with citrus flavonoids may be a good strategy to improve their welfare. Flavonoid supplementation may modify the behavior and rumen inflammatory response of fattening bulls, and this could be related to the concentrate presentation (mash or pellet) form. In the present study, 150 Holstein bulls (183.0 ± 7.53 kg BW and 137 ± 1.8 d of age) were randomly allocated to one of eight pens and assigned to control (C) or (BF) (Citrus aurantium, Bioflavex CA, HealthTech Bio Actives, Spain, 0.4 kg per ton of concentrate of Bioflavex CA, 20% naringin). Concentrate (pellet) intake was recorded daily, and BW and animal behavior fortnightly. Animals were slaughtered after 168 d of study, and ruminal epithelium samples were collected for gene expression analyses. Treatment did not affect animal performance; however, BF supplementation reduced agonistic interactions and oral non-nutritive behaviors and increased the time devoted to eating concentrate and ruminating activity (p < 0.05). The gene expression of some genes in the rumen epithelium was greater or tended to be greater in BF than C bulls (bitter taste receptor 16, cytokine IL-25, β-defensin; p < 0.10; pancreatic polypeptide receptor 1 and tumor necrosis factor alpha; p < 0.05). In conclusion, flavonoid supplementation modifies the expression of genes in the rumen epithelium that could be related to inflammation and animal behavior modulation. [ABSTRACT FROM AUTHOR]