Effects of Feed Composition in Different Growth Stages on Rumen Fermentation and Microbial Diversity of Hanwoo Steers

SIMPLE SUMMARY: Ruminants are a major source of greenhouse gases and environmental pollution, which are exacerbated by excessive feeding because excess nutrients are excreted. Since digestion in ruminants is aided by both microorganisms and digestive enzymes, information on gut microbiota in ruminants is vital for assessing the adequate amount of feed supply. The growth of Hanwoo steer is divided into three stages, and specific nutrients are provided in each stage. Growth stages are a major factor influencing rumen fermentation by microorganisms. In this study, changes in fermentation characteristics and microbial community during the growth stages were investigated. Our findings show that there were differences in the levels of total volatile fatty acids, propionate, and valerate. In addition, we found that the relative content of some bacteria and fungi changed with the growth stage. This study improves our understanding of rumen fermentation and changes in the gut microbiota during the various growth stages of Hanwoo steer. ABSTRACT: Ruminants are a major source of greenhouse gas emissions, and information on ruminant fermentation and microorganisms is essential to understand ruminant digestion, which is associated with environmental pollution. The present study investigated rumen fermentation and microbial diversity according to the three different growth stages of four Hanwoo steers: growing (12 months, G), early fattening (18 months, EF), and late fattening (25 months, LF). No significant differences were observed in rumen pH and ammonia nitrogen among growth stages. Total volatile fatty acids were significantly higher and propionate and valerate significantly lower in G than in EF and LF (p < 0.05). Ten bacterial phyla were detected, including Firmicutes (47.5–53.5%) and Bacteroidetes (28.4–31.7%), which accounted for 79.2–82.3% of the total bacteria. Prevotella accounted for the highest proportion (31.6–42.6%) of all bacteria in this study but did not differ significantly among the different growth stages. Metaprevotella abundance was significantly higher in G than in the other treatments (p < 0.05). In addition, Paraprevotella tended to be higher in LF than in the other treatments (p = 0.056). Given the differences in the genera of microorganisms with relatively low abundance, additional experiments are needed to determine the effect on fermentation.