3.1. Body Weight and Average Daily Gain
Table 2 shows the effects of
C. butyricum on the body weight, ADG, and FCR of Hanwoo calves. Birth and weaning weights were not significantly affected by
C. butyricum supplementation (birth weight,
p = 0.259; weaning weight,
p = 0.896). Although ADG showed an increasing trend with increasing levels of
C. butyricum supplementation, ADG was not significantly different (
p = 0.835) among the groups, with values ranging from 0.86 to 0.91 kg/day.
Research findings indicate that the effects of
C. butyricum supplementation on growth performance vary among different livestock species and management conditions. For example, Li et al. [
26] found that
C. butyricum supplementation significantly increased DMI and body weight in cows, whereas Zhang et al. [
27] found no significant changes in the growth indicators in goats. These differences suggest that the effects of
C. butyricum supplementation may be affected by various factors, including species, current health status, feeding environment, and composition of gut microbes.
In the present study, C. butyricum supplementation did not considerably affect growth performance parameters, including birth weight, weaning weight, and ADG, in Hanwoo calves. This may reflect the limited direct impact of probiotics on growth, although it is also possible that factors such as dosage, trial duration, and sample size may have influenced the statistical outcomes. Therefore, future studies should adopt longer feeding trials, include a range of dosage levels, and ensure adequate sample sizes to determine the effect of C. butyricum on the growth performance of calves.
3.2. Blood Biochemical Parameters
Table 3 shows the effects of
C. butyricum supplementation on blood metabolite levels. Blood amylase was markedly lower in the CB1 and CB2 groups compared with the CON group (overall,
p < 0.01), whereas the CB3 group showed intermediate values. Triglyceride concentrations also exhibited a quadratic response (
p = 0.036), being higher in the CB1 and CB2 groups but declining in the CB3 group. Other blood metabolites, including cholesterol, NEFA, total protein, albumin, and liver enzymes (AST, GGT, ALP), did not differ significantly among groups (
p > 0.05).
Amylase is a digestive enzyme primarily secreted by the pancreas and salivary glands and is present at high concentrations in digestive fluids, with small amounts detected in the blood and urine [
35,
36,
37]. In ruminant species, salivary secretion of amylase is negligible, distinguishing them from non-ruminant animals [
38,
39,
40]. Circulating amylase levels are commonly utilized as biomarkers for pancreatic and gastrointestinal functions and have been employed in the clinical evaluation of pathological conditions such as pancreatitis and gastroenteritis.
Recently, there has been growing recognition that indicators such as serum amylase should be interpreted within the framework of host–microbiota interactions, particularly along the gut–pancreas axis. This concept has led to increasing research interest on the bidirectional physiological mechanisms underlying this axis. Among various microbial metabolites, VFAs have been reported to influence pancreatic function [
12,
41]. Particularly, sodium butyrate modulates pancreatic function by suppressing nuclear factor-kappa B (NF-κB) activity and inhibiting histone deacetylase, thereby regulating inflammation and fibrosis [
41,
42].
In the present study, the observed decrease in serum amylase concentration following C. butyricum supplementation suggests that changes in the intestinal environment indirectly influence pancreatic function via the gut–pancreas axis. However, ruminants possess a complex microbial ecosystem within the rumen and distinct microbial communities along the intestinal tract, resulting in a multilayered microbiota structure. Because of this complexity, the physiological link between microbial populations and digestive organs, including the pancreas, is highly intricate and not well understood in ruminants. Furthermore, as this study did not directly assess pancreatic responses or the expression of genes related to pancreatic function, definitive conclusions regarding the underlying mechanisms of the observed phenomenon cannot be drawn.
However, previous studies have reported that metabolites, such as VFAs, can improve intestinal homeostasis, modulate immune and metabolic functions, and potentially affect extraintestinal organs such as the pancreas [
23,
24,
25]. Our findings may provide foundational evidence for future investigations into microbiota–organ interactions mediated by the gut–pancreas axis in ruminants. However, further studies are required to elucidate the precise biological mechanisms underlying this process.
Table 4 shows the hematological parameters of calves fed diets supplemented with different
C. butyricum doses. No significant differences were observed among the groups for most parameters. In contrast, the white blood cell count tended to differ (overall,
p = 0.078; linear,
p = 0.072) and showed a significant quadratic trend (
p = 0.014), increasing in the CON group, decreasing in the CB1 and CB2 groups, and then increasing in the CB3 group. Neutrophil counts were significantly higher in the CON group than in the CB1 and CB2 groups (overall,
p = 0.033; quadratic response [
p = 0.006]), with the CB3 group showing intermediate values.
Notably, differences in WBC and neutrophil counts were observed among the treatment groups. Although the mechanisms of
C. butyricum in immune regulation and inflammation were not directly investigated in the present study, previous studies have reported that
C. butyricum exhibits anti-inflammatory and immunomodulatory effects by regulating cytokine production and immune cell differentiation [
43,
44]. Specifically,
C. butyricum enhances the host immune system by upregulating pro-inflammatory cytokines such as IL-8, IL-6, and TNF-α, and also exerts beneficial effects through the production of anti-inflammatory cytokines such as IL-10 [
45,
46]. Additionally, some studies have shown that
C. butyricum may suppress the NF-κB signaling pathway, thereby regulating the transcription of inflammatory genes and inhibiting excessive inflammatory responses and immune cell activation [
42,
47]. Similarly, Zhang et al. [
21] reported that dietary supplementation with
C. butyricum upregulated TNF-α and IL-4 concentrations in the jejunal mucosa of broiler chickens compared with those in the control group. Additionally, studies in piglets have shown that 0.4%
C. butyricum supplementation increases the relative mRNA expression of TLR2 and IL-10 in the ileum. Although these findings were obtained from monogastric animals, they suggest that
C. butyricum supplementation may modulate immune responses by balancing pro-inflammatory and anti-inflammatory signaling pathways, contributing to improved intestinal immune regulation. However, a limitation of this study is that specific immune-related markers, such as cytokine levels, were not analyzed. Future studies should incorporate comprehensive assessments of inflammation-related markers and hematological parameters to elucidate the immunomodulatory effects of
C. butyricum.
Furthermore, the effects of
C. butyricum supplementation on blood acid–base parameters are presented in
Table 5. No significant differences were observed in pH levels among the groups (overall,
p > 0.05). However, there was a significant increase in TCO
2, HCO
3, and base excess and a decrease in anion gap in the CB3 group compared with those in the CON group (overall,
p < 0.01). Additionally, these parameters showed clear linear responses across the supplementation levels (linear,
p < 0.01).
TCO
2, HCO
3−, and pH are critical indicators of systemic acid–base balance. Among these, HCO
3−—which constitutes the major component of TCO
2—functions as a primary physiological buffer and is widely recognized as a key marker for assessing metabolic acid–base disorders, including metabolic acidosis and alkalosis. The bicarbonate buffering system not only maintains systemic acid–base homeostasis but also plays a vital role in regulating the ruminal environment of gastrointestinal regions with active microbial fermentation [
48,
49,
50]. In the rumen, the accumulation of VFAs during fermentation predisposes the environment to acidification; however, HCO
3− secreted via saliva contributes to the neutralization of this acidity, thereby stabilizing ruminal pH. Consequently, HCO
3− and VFAs in the rumen are essential for sustaining microbial homeostasis and acid–base equilibrium. Moreover, localized buffering activity in the rumen may have systemic implications, potentially influencing circulating HCO
3− concentrations [
51,
52,
53].
In the present study,
C. butyricum supplementation significantly increased both serum HCO
3− concentration and blood pH. Although
C. butyricum was not directly investigated in previous ruminal acidosis models, the existing literature has demonstrated that co-administration of probiotics and sodium bicarbonate ameliorates acid–base disturbances and elevates blood pH under experimentally induced acidosis [
14]. Collectively, these findings imply that probiotics may contribute to mitigating acidic ruminal conditions and restoration of systemic pH by modulating SCFA production and enhancing ruminal buffering capacity.
Therefore, these findings suggest that C. butyricum supplementation may influence the profile of ruminal fermentation metabolites, thereby enhancing the activity of the bicarbonate buffering system and promoting ruminal pH stability, which may contribute to the maintenance of systemic acid–base homeostasis.
3.4. Rumen and Fecal Alpha Diversity and Microbiota
In this study, we investigated the alpha diversities of the rumen and fecal microbiota of calves fed
C. butyricum-supplemented diets (
Table 7). No significant differences were observed among the treatment groups for any of the indices evaluated, including the ASVs, Chao1, Shannon, and Gini–Simpson indices, in both the rumen and feces (
p > 0.05). In the rumen, the Shannon index ranged from 5.84 to 6.70, and the Gini–Simpson index remained below 0.05 across all groups. In fecal samples, the Shannon index was consistently above 5, and the Gini–Simpson index ranged from 0.059 to 0.121, suggesting a more even microbial community than that in the rumen. Overall, these values indicate that the ecological stability of microbial diversity and evenness was preserved despite probiotic supplementation.
Additionally, the composition of rumen microbiota is summarized in
Figure 1 and
Tables S1 and S2. At the phylum level, Firmicutes and Bacteroidetes were predominant across all groups, accounting for approximately 40–51% and 41–55% of the total sequences, respectively. Other phyla, including Actinobacteria, Fibrobacterota, Cyanobacteria, and Spirochaetota, were detected at lower relative abundances (<5%), with no significant differences among the groups (
p > 0.05). Additionally, similarities and differences in community structure among groups were determined and visualized using principal coordinate analysis (PCoA) based on beta diversity metrics (
Supplementary Figure S1).
At the genus level, Prevotella was the most dominant taxon, with a relative abundance ranging from 24.13% in the CON group to 45.93% in the CB2 group. Although these differences were not statistically significant (p = 0.238), higher abundances were observed in all C. butyricum-supplemented groups than in the CON group. Similarly, Muribaculaceae abundance increased from 2.95% in the CON group to 7.52% in the CB3 group, showing a near-significant trend (p = 0.056). In contrast, the relative abundance of the Rikenellaceae RC9 gut and F082 decreased notably in the supplementation groups.
Prevotella plays a central role in carbohydrate and hydrogen metabolism in the rumen of ruminants. Members of this genus possess a wide range of enzymes that are capable of degrading various polysaccharides, thereby contributing to VFA production [
55,
56]. Additionally, some species have been reported to be proteolytic bacteria that produce cysteine proteases and collagen-degrading enzymes [
55,
57]. Although the functional importance of
Prevotella has been well demonstrated in previous studies, the present study did not include metabolomic analyses, such as VFA profiling, to directly assess its metabolic activity or its contribution to fermentation. In future studies, metabolomic approaches should be integrated to quantitatively evaluate the effects of
C. butyricum on rumen fermentation parameters and clarify its relationship with microbial composition.
In the present study, an increasing trend in the relative abundances of not only
Prevotella but also
Muribaculaceae was observed in the
C. butyricum-supplemented groups.
Muribaculaceae, primarily belonging to the phylum Bacteroidetes, is a group of commensal bacteria that produces enzymes specialized in degrading dietary polysaccharides and fibers [
58,
59]. Their abundance has been shown to vary in response to the dietary forage-to-concentrate ratio, specific feed ingredients, and probiotic supplementation [
60,
61] and they may interact with fermentation conditions and rumen microbial community structure in response to environmental changes [
62,
63].
Previous studies have reported that dietary supplementation with
C. butyricum increases the relative abundance of genera such as
Prevotella,
Ruminococcaceae, and
Megasphaera and promotes the production of VFAs [
23,
27]. Moreover, appropriate levels of
C. butyricum supplementation can facilitate probiotic colonization of the gastrointestinal tract, enhance gut barrier function, and protect against pathogenic bacterial invasion [
24]. In the present study, all groups were fed an identical total mixed ration and maintained under standardized housing conditions to minimize the effects of external factors. Under these controlled conditions, the observed increase in the relative abundances of
Prevotella and
Muribaculaceae in the
C. butyricum-supplemented groups suggests that this strain may indirectly promote the growth of these microbes by improving the gut microenvironment, reducing the abundance of competing bacteria, and altering the availability of substrates.
Furthermore, the composition of fecal microbiota is summarized in
Figure 2 and
Tables S3 and S4. At the phylum level, Firmicutes was the most dominant taxon across all groups, accounting for approximately 63–71% of the total sequences, followed by Proteobacteria (4.8–18.2%), Bacteroidetes (10.6–31.9%), and Actinobacteria (5.6–11.8%). Other phyla, such as Cyanobacteria and Verrucomicrobiota, were present in lower proportions (<5%), with no significant differences among the treatment groups (
p > 0.05). Moreover, similarities and differences in community structure among groups were determined and visualized using PCoA based on beta diversity metrics (
Supplementary Figure S2).
Similarly, no significant differences (p > 0.05) were observed in the relative abundances of most genera among the treatment groups. However, several genera showed trends in response to C. butyricum supplementation. Escherichia–Shigella showed a decreasing trend, with the highest abundance observed in the control group (13.78%) and progressively lower values in the treatment groups, reaching 2.31% in CB3. In contrast, increasing trends were observed for genera such as Bacteroides, Faecalibacterium, and Gastranaerophilales in the C. butyricum-supplemented group. Although Tyzzerella showed a significant increase in CB3, its overall abundance was low, suggesting that its biological relevance is limited.
The reduced abundance of
E.
coli in the
C. butyricum-supplemented groups suggests that the probiotic suppresses potentially pathogenic bacteria. The
Escherichia–
Shigella genera includes several strains known for their pathogenicity, such as enterotoxigenic
E.
coli (ETEC), which is a major cause of diarrhea and gastrointestinal disturbances in animals [
64,
65]. These pathogens can cause diarrhea, dehydration, and impaired nutrient absorption, ultimately leading to reduced growth and increased mortality in neonatal calves. Therefore, the observed decrease in
E.
coli abundance following
C. butyricum supplementation may be associated with its benefits in reducing the frequency and severity of calf diarrhea.