The Combination of Phytogenic in High-Starch Diets for Growing Cattle and Its Effects on Immunomodulation, Fermentation, Microbiota, and Performance.

High-starch diets can impair gastrointestinal homeostasis in growing cattle by promoting dysbiosis, inflammation, and oxidative stress. This study evaluated the effects of a multi-active phytogenic blend containing Macleaya cordata extract, essential oils, live yeast, silymarin, bentonite, and beta-glucans on performance, inflammatory and oxidative biomarkers, volatile fatty acids (VFA) production, and microbiota composition in calves fed a high-starch diet. Twenty-eight 5-month-old Holstein males were assigned to a control diet (control group) or the same diet with addition of 15 g/animal/day of the additive (Phytogenic group) for 70 days. Calves fed with additives had greater weight gain and average daily gain (p = 0.05), whereas dry matter intake and feed efficiency were unchanged. The additive reduced total leukocytes and lymphocytes number, decreased of biomarker in serum as TNF-α, IL-1, C-reactive protein, ferritin, haptoglobin, creatine kinase, cholinesterase, TBARS, and reactive oxygen species. These animals of the Phytogenic group also have increased IL-10, IgA, IgG, total protein, and globulin concentrations compared to the control group. Ruminal and fecal VFA concentrations were higher in animals of Phytogenic group, including acetate, propionate, and butyrate. In addition, intake of additives increased ruminal microbial richness and altered beta diversity, indicating a marked shift in rumen microbial structure, while altering the relative abundance of specific taxa associated with high-starch feeding. These findings indicate that this phytogenic blend improves growth performance and exerts immunomodulatory, antioxidant, and microbiota-modulating effects, supporting its use as a nutritional strategy to mitigate the metabolic challenges associated with high-starch diets in growing cattle.