Effect of Diet Supplementation with Two Yeast Cultures on Rumen Fermentation Parameters and Microbiota of Fattening Sheep In Vitro

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 (<i>p</i> < 0.05) in cumulative gas production and microbial protein content, while a significant decrease (<i>p</i> < 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 (<i>p</i> < 0.05) than that in the XP group. Additionally, the content of valeric acid and isobutyric acid in the XP group was significantly higher (<i>p</i> < 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 (<i>p</i> > 0.05); however, the addition of XP significantly reduced (<i>p</i> < 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.