This trial aimed to investigate the effects of compound essential oils (EO) on the fattening performance, blood physiological–biochemical indices, and intestinal microbiota in late-fattening Simmental crossbred bulls. Twenty healthy Simmental crossbred bulls (Simmental × Charolais × Angus) with similar initial body weights of 442 (±72.49) kg were randomly divided into two groups: a control group (basal diet, CON group) and a compound essential oil group (basal diet + 16 g/head/day, EO group). The trial included a 14-day pre-feeding period and a 42-day experimental period, totaling 56 days. The results showed the following: (1) The EO group exhibited a significantly higher average daily gain (ADG), immunoglobulin A (IgA), immunoglobulin G (IgG), total superoxide dismutase (T-SOD), glutathione peroxidase (GSH-PX), glucose (GLU), dopamine (DA), basophil count (Baso), mean corpuscular hemoglobin (MCH), and platelet distribution width (PDW) compared to the CON group (p < 0.05), while the mean corpuscular volume (MCV) was significantly lower (p < 0.05). (2) Although the compound essential oil supplementation did not alter the relative abundance of major intestinal microbial taxa, it significantly improved the intestinal microbiota structure (p < 0.05), increased fiberdegrading microbiota, and promoted short-chain fatty acid production. (3) The relative abundance of the intestinal microbiota norank_f__UCG-010 showed significant positive correlations with ADG, GSH-PX, IgG, DA, T-SOD, GLU, IgA, and Baso (p < 0.05), while Christensenellaceae_R-7_group abundance was positively correlated with ADG, IgA, and Baso (p < 0.05). In conclusion, the compound essential oil enhances healthy and efficient fattening in beef cattle by improving the intestinal microbial structure, increasing beneficial bacteria, regulating the nutrient metabolism through key bacterial genera, and enhancing the immune function, antioxidant capacity, and energy metabolism levels.

Oxidative stress can impair calf health and performance, and oregano essential oil (OEO) has potential antioxidant activity. This study evaluated dietary OEO in a diquat-induced oxidative-stress model using 24 weaned male Holstein calves assigned to a 2 × 2 factorial design with OEO supplementation (0 or 8 g/day) and diquat challenge (saline or 8 mg/kg body weight). Growth performance, serum biochemical, antioxidant, oxidative-damage and inflammatory indices, hepatic Nrf2-related antioxidant gene expression, rumen microbiota, and rumen metabolomics were assessed. The results showed that growth responses were phase dependent: OEO increased overall average daily gain (ADG) and dry matter intake (DMI), whereas diquat reduced overall ADG; no OEO × diquat interaction was detected for overall growth outcomes. The OEO group had higher serum SOD, GSH-Px, and T-AOC levels and lower levels of selected oxidative-damage and inflammatory indices, although the responses varied by endpoint and time point. Hepatic RT-qPCR showed treatment-associated differences in Keap1, HMOX1, GCLC, GCLM, and SOD1 expression. Rumen 16S rRNA gene sequencing showed treatment-associated shifts in microbial community composition, whereas PICRUSt2 predicted differences in microbial functional potential, including amino-acid-metabolism-related functions. Untargeted metabolomics identified treatment-associated differences in rumen metabolite profiles and KEGG pathway-enrichment patterns. Correlation analysis indicated that OEO-associated characteristic metabolites were positively correlated with antioxidant markers and negatively correlated with oxidative-damage and pro-inflammatory indices. Overall, OEO supplementation was associated with improved antioxidant status and distinct rumen microbial-metabolic profiles in diquat-challenged calves, supporting its potential as a natural feed additive, while the involvement of specific molecular pathways requires further validation.

This study aimed to evaluate how dietary yeast cell wall (YCW) supplementation in the starter feed affects ruminal fermentation parameters, microbial community composition, and metabolite profiles in early-weaned Simmental calves. Twenty-four newborn Simmental heifer calves (initial body weight: 37.53 ± 2.50 kg) were randomly assigned based on birth date sequence into the experimental group and the control group (12 calves per group). Calves in the experimental group (YCW) received a daily supplement of 5 g/head/day of yeast cell wall in the starter diet, whereas those in the control group (CON) received no supplementation. The experimental period lasted for 100 days, with weaning conducted at 70 days of age. On day 70, rumen fluid samples were randomly collected from six calves per group for analysis of rumen fermentation parameters, microbial community composition, and metabolomic profiles. (1) YCW supplementation significantly increased ruminal butyrate concentration and the relative abundance of the genus Ruminococcus (p < 0.05); (2) Metabolomic analysis identified 43 differential metabolites (20 upregulated and 23 downregulated), with nucleotide metabolism–related compounds such as guanylic acid and deoxycytidine monophosphate being prominently enriched (p < 0.05); (3) Spearman correlation analysis further revealed positive associations between Ruminococcus and both butyrate levels and selected upregulated metabolites, including guanylic acid (p < 0.05). Dietary yeast cell wall supplementation enhanced ruminal fermentation in early-weaned Simmental calves by increasing butyrate concentration and altering the ruminal microbiota and metabolome. Enrichment of Ruminococcus and nucleotide-associated metabolites, with positive correlations to butyrate, indicates a coordinated shift in the microbiota–metabolite axis. These findings support YCW as an effective nutritional strategy to promote rumen development and health during the early weaning period.