Simple SummaryHigh-concentrate diets can damage the barrier function of the rumen epithelium and lead to inflammation, which may result in digestive and metabolic disorders and growth retardation in lambs. The phenolic phytonutrients, such as cinnamaldehyde, eugenol, and capsicum oleoresin, have been suggested to act as rumen modifiers. Our results show that a blend of cinnamaldehyde, eugenol, and capsicum oleoresin (CEC) supplementation improved the growth performance and rumen health of lambs fed a high-concentrate diet, which could be due to reducing inflammation and apoptosis, protecting barrier function, and modulating the bacterial community.AbstractWe investigated the effects of CEC on the fermentation characteristics, epithelial gene expression, and bacterial community in the rumen of lambs fed a high-concentrate diet. Twenty-four 3-month-old female crossbred lambs with an initial body weight of 30.37 ± 0.57 kg were randomly allocated to consume a diet supplemented with 80 mg/kg CEC (CEC) or not (CON). The experiment consisted of a 14 d adaptation period and a 60 d data collection period. Compared with the CON group, the CEC group had higher ADG, epithelial cell thickness, ruminal butyrate proportion, and lower ammonia nitrogen concentration. Increases in the mRNA expression ofOccludinandClaudin-4, as well as decreases in the mRNA expression of apoptotic protease activating factor-1 (Apaf-1), cytochrome c (Cyt-C),Caspase-8,Caspase-9,Caspase-3,Caspase-7, and toll-like receptor 4 (TLR4), were observed in the CEC group. Moreover, CEC treatment also decreased the concentration of IL-1β, IL-12, and TNF-α. Supplementation with CEC altered the structure and composition of the rumen bacterial community, which was indicated by the increased relative abundances of Firmicutes, Synergistota,Rikenellaceae_RC9_gut_group,Olsenella,Schwartzia,Erysipelotrichaceae_UCG-002,Lachnospiraceae_NK3A20_group,Acetitomaculum,[Eubacterium]_ruminantium_group,Prevotellaceae_UCG-004,Christensenellaceae_R-7_group,Sphaerochaeta,Pyramidobacter, and[Eubacterium]_eligens_group, and the decreased relative abundances of Acidobacteriota, Chloroflexi, Gemmatimonadota, andMND1. Furthermore, Spearman correlation analysis revealed that the altered rumen bacteria were closely correlated with rumen health-related indices. Dietary CEC supplementation improved growth performance, reduced inflammation and apoptosis, protected barrier function, and modulated the bacterial community of lambs fed a high-concentrate diet.