Effects of Replacing Whole-Crop Maize Silage with Pennisetum giganteum Silage on the Rumen Microbial Community in Beef Cattle

This study evaluated the effects of replacing whole-crop maize silage with varying proportions of Pennisetum giganteum silage on rumen fermentation, microbial composition, and metabolic function in beef cattle. A single-factor completely randomized design was employed using 50 healthy crossbred Simmental cattle aged 11–12 months (average body weight: 251.08 ± 51.54 kg). Animals were randomly assigned to five groups, with 10 replicates per group and one animal per replicate. Diets contained 0% (Group A), 25% (Group B), 50% (Group C), 75% (Group D), or 100% (Group E) Pennisetum giganteum silage replacing whole-crop maize silage over a 67-day feeding period, including a 7-day adaptation phase. Rumen fluid samples were collected via rumen catheter at the end of the trial to assess bacterial diversity and functional characteristics. Increasing the proportion of Pennisetum giganteum silage resulted in quadratic changes in volatile fatty acids (VFAs) and propionate (PA) concentrations (p < 0.05), while ammonia nitrogen (NH3-N) increased linearly (p < 0.05). No significant differences were observed in α- or β-diversity among groups (p > 0.05). Group C exhibited significantly higher relative abundances of Verrucomicrobiota and Prevotellaceae_UCG_003 compared with the other groups (p < 0.05). At the phylum level, Proteobacteria increased linearly, whereas Spirochaetota decreased linearly; at the genus level, Treponema decreased linearly (p < 0.05). LEfSe analysis indicated enrichment of g__Prevotellaceae_UCG_003 and o__WCHB1_41 in Group C, while the relative abundances of f__Enterobacteriaceae and g__Citrobacter were elevated in Group E. Under the conditions of this study, replacing 50% of whole-crop maize silage with Pennisetum giganteum silage enhanced rumen fermentation efficiency and modulated key microbial populations in beef cattle.