Calf diarrhea is a common early-life disorder that adversely affects growth, oxidative balance, immune function, and intestinal microbiota, thereby compromising health and production performance. This study systematically investigates the effects of naturally occurring diarrhea in 7-day-old suckling calves on oxidative stress, immune responses, intestinal barrier integrity, and gut microbiota structure and function. Fecal scores, serum antioxidant and immune indices, and intestinal permeability markers were measured, and fecal samples were subjected to metagenomic sequencing. Diarrhea-affected calves exhibited higher fecal scores, increased oxidative stress indicated by reduced total antioxidant capacity, elevated lipid peroxidation, and altered antioxidant enzyme activities. Humoral immunity was impaired, inflammatory responses were dysregulated, and intestinal barrier function was disrupted. Gut microbial diversity declined, showing a depletion in health-associated taxa and the enrichment of opportunistic pathogens. Correlation analyses revealed that pathogenic bacteria abundance positively associated with diarrhea severity, oxidative stress, inflammation, and barrier disruption, while beneficial genera correlated with antioxidant and immune function. Functional profiling indicated a microbial shift from amino acid metabolism and antioxidant homeostasis toward carbohydrate and energy metabolism under diarrheic conditions. These findings highlight the pivotal role of gut microbiota dysbiosis in diarrhea pathogenesis and provide a foundation for developing microbiome-targeted interventions to improve calf health.

Diarrhea in suckling calves is associated with impaired growth, oxidative stress, immune dysfunction, and intestinal microbial dysbiosis. This study evaluated the effects of compound yeast culture (CYC) supplementation on growth performance, fecal characteristics, antioxidant capacity, immune function, and gut microbiota in diarrheic Holstein calves. Thirty-six approximately 7-day-old calves were enrolled, including 12 healthy calves (CON) and 24 diarrheic calves randomly assigned to a diarrhea group (DIA) or a CYC-supplemented group (DIA-YC; 50 g/d for 30 days). The experimental period lasted 60 days. Compared with the DIA group, calves in the DIA-YC group showed significantly higher average daily feed intake and average daily gain (ADG) during days 31–60 and across the entire period (p < 0.05), with a trend towards increased body weight. Fecal scores were significantly elevated in diarrheic calves during the early and mid-stages but were markedly reduced by CYC supplementation from days 7 to 30; no significant difference was observed between DIA-YC and CON during days 16–30 (p > 0.05). Diarrheic calves exhibited oxidative stress, characterized by decreased total antioxidant capacity (T-AOC) and increased malondialdehyde (MDA). CYC supplementation significantly increased T-AOC, superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px) activities, while reducing MDA levels (p < 0.05). Immune analysis showed higher serum IgG and IL-10 levels and lower TNF-α levels in the DIA-YC group, along with improved intestinal barrier indicators, including diamine oxidase (DAO) activity and endotoxin levels. Metagenomic analysis revealed that diarrhea reduced microbial richness and diversity and altered community structure, whereas CYC partially restored microbial diversity and increased beneficial genera such as Prevotella, Coprococcus, Ruminococcus, and Parabacteroides. Functional analysis indicated that CYC enhanced pathways related to immune regulation, energy metabolism, and antioxidant function. CYC supplementation alleviates oxidative stress and immune dysfunction by modulating gut microbiota, thereby improving growth performance and reducing diarrheal severity in calves.