Yeast culture can improve ruminant health and reduce economic losses in intensive farming, but as a non-standardized product in China, its quality and efficacy vary significantly. In this study, a self-developed yeast culture was compared with a commercially available product using in vitro rumen fermentation and amplicon-based high-throughput sequencing to evaluate its effects on rumen fermentation parameters, microbial diversities, and community compositions in Hu sheep. The aim was to validate the efficacy and mechanisms of the self-developed yeast culture, produced with simplified raw materials and processes, on rumen function. The experiment was divided into four groups. In each 60 mL fermentation solution, the following treatments were added: 0.00 g high-concentrate diet (CK1 group, blank control), 0.40 g high-concentrate diet (CK2 group, basal diet control), 0.40 g high-concentrate diet supplemented with 5% XP yeast culture (XP group), and 0.40 g high-concentrate diet supplemented with 5% YC yeast culture (YC group). Gas production was measured every 4 h during fermentation. At the end of fermentation, pH, ammonia nitrogen, microbial protein, volatile fatty acids, and ruminal microbiota were determined. The results demonstrated the following. Compared to the CK2 group, both the XP and YC groups exhibited a significant increase (p < 0.05) in cumulative gas production and microbial protein content, while a significant decrease (p < 0.05) was observed in acetic acid content and the acetate-to-propionate ratio. The microbial protein content in the YC group was significantly higher (p < 0.05) than that in the XP group. Additionally, the content of valeric acid and isobutyric acid in the XP group was significantly higher (p < 0.05) compared to the other groups. The microbial community sequencing results revealed that the addition of yeast culture did not affect the alpha diversity index of rumen bacteria (p > 0.05); however, the addition of XP significantly reduced (p < 0.05) the richness of rumen fungal communities. At the phylum and genus levels, the relative abundance of multiple functional bacteria improved after adding YC. In summary, under the conditions of in vitro rumen fermentation with high-concentrate diets, adding 5% XP and YC yeast cultures both promoted rumen fermentation. The rumen fermentation type changed from the acetic acid type to the propionic acid type, which regulated rumen microbial composition and thereby improved dietary digestion efficiency. Notably, YC significantly increased the relative abundance of functional microbial communities compared to XP. These findings provide a theoretical and practical foundation for optimizing the large-scale breeding of Hu sheep.

To explore the appropriate supplementation rate of yeast culture (YC) in Kazakh sheep during fattening, the effects of different YC supplementation rates on rumen fermentation parameters and microbial community were studied through in vitro rumen fluid fermentation experiments. A 0.40 g high-concentrate diet was used as the fermentation substrate, and five groups were added with YC at 0% (CK), 1.25% (YC1), 2.5% (YC2), 3.75% (YC3) and 5% (YC4) of dietary dry matter, respectively. Anaerobic fermentation was carried out for 48 h in 60 mL fermentation broth. The results showed that the 48 h GP and microbial crude protein (MCP) concentration in all YC supplementation groups were significantly higher than those in the CK group (p < 0.05). The concentrations of total volatile fatty acids (TVFA) and propionate in the YC1 and YC2 groups were significantly increased and the A/P ratio in the two groups was significantly decreased (p < 0.05). The Multi-factor Comprehensive Evaluation Index (MFAEI) calculation indicated that 1.25% was appropriate. The YC1 and YC2 groups significantly increased the richness and diversity of rumen bacterial communities (Chao1 and Shannon indices, p < 0.05), and significantly increased the relative abundance of Bacteroidota and NK4A214_group (p < 0.05), while significantly decreasing the relative abundance of the potential pathogenic bacterium Campylobacter (p < 0.05). Ustilago abundance was significantly suppressed in all the YC-supplemented groups (p < 0.05). The most effective YC supplementation rate among the tested doses was 1.25% according to the MFAEI and key microbial indicators. The results suggest that dietary supplementation of 1.25% YC (dry matter basis) may beneficially modulate rumen fermentation parameters under in vitro conditions, providing a reference for further in vivo studies on its application in fattening Kazakh sheep.

Rumen development and microecological homeostasis are critical for post-weaning health in lambs; however, effective nutritional strategies combining probiotics and antimicrobial peptides remain underexplored. This study investigated the effects of graded doses of a combined additive (Candida utilis + antimicrobial peptide (AMP)) on rumen development and microecology in weaned Hu lambs. Forty healthy pre-weaning male lambs (initial body weight: 15.77 ± 1.25 kg; age: 42.98 ± 1.80 d) were randomly assigned to four groups (n = 10): a control group (CK, fed milk/basal diet), and three treatment groups receiving the same milk/basal diet supplemented with graded doses of the combined additive, based on feed intake (LCB: AMP 0.05 g/kg + C. utilis 0.1 g/kg, MCB: AMP 0.05 g/kg + C. utilis 0.8 g/kg, HCB: AMP 0.05 g/kg + C. utilis 1.5 g/kg) over a 40-day period starting from day 6. At the end of the trial, six lambs per group were randomly selected for sampling for analyses of rumen morphology, fermentation parameters, 16S rRNA amplicon sequencing, and untargeted metabolomics. Results showed that, compared with CK, (1) MCB and HCB increased papilla height, and HCB also increased papilla width (p < 0.05); papilla width and muscular layer thickness showed significant linear increases with increasing additive dose (p < 0.05). (2) All treatments linearly reduced ruminal pH (p < 0.05), although all values remained within the normal physiological range; meanwhile, all treatment groups showed elevated levels of microbial protein (MCP), butyrate, valerate, and isovalerate; MCP also increased linearly with dose (p < 0.05); LCB decreased the acetate-to-propionate ratio(A:P), and this ratio exhibited a significant quadratic trend across treatment groups (p < 0.05). (3) HCB increased the α-diversity indices and reduced Methanobacteriota abundance; LCB elevated Bacillota; and all treatments decreased Spirochaetota, Synergistota, and Cyanobacteriota but increased Actinomycetota, Verrucomicrobiota, and Thermodesulfobacteriota (p < 0.05). (4) The number of differential metabolites and KEGG pathway enrichments gradually increased with increasing doses of the combined additive, with HCB exerting the most extensive modulatory effects on signaling and neural pathways. (5) Volatile fatty acids, Bacteroidota, and Xylanibacter were positively correlated with rumen development parameters (p < 0.05). Collectively, dietary supplementation with the combined additive improved rumen fermentation, microbial community structure, and metabolic profiles, with the high-dose (HCB) regimen showing the most effects on select parameters, whereas the low-dose (LCB) regimen exhibited advantages in specific fermentation traits. These findings provide preliminary evidence for the potential application of this combined additive in post-weaning lamb diets.