Effects of 5-Hydroxymethylfurfural on In Vitro Rumen Fermentation Characteristics and Bacterial Community Structure of Diets with Different Concentrate-to-Roughage Ratios

To investigate the effects of 5-hydroxymethylfurfural (5-HMF) on ruminal fermentation characteristics and bacterial community structure, 5-HMF was supplemented at the levels of 0, 7500, and 30,000 mg/kg DM under two dietary concentrate-to-roughage ratios (20:80, T1; 80:20, T2) in in vitro rumen incubation, mainly focusing on gas production, fermentation parameters, nutrient digestibility and bacterial community. The results showed that 24 h gas production (GP24), 72 h gas production (GP72), the gas production rate (c) and dry matter digestibility (DMD) in T2 were higher than those in T1 (p < 0.01). With increasing levels of 5-HMF supplementation, GP72 and asymptotic gas production (B) increased linearly and quadratically (p < 0.05). T2 also had a higher butyrate concentration and a lower acetate/propionate (A/P) ratio than T1 (p < 0.05). With the increase in 5-HMF supplementation level, ruminal ammonia nitrogen (NH3-N) concentration showed a quadratic response. In contrast, the concentrations of total volatile fatty acids (TVFAs), acetate and isobutyrate decreased linearly. Propionate concentration showed linear and quadratic decreasing trends, while the A/P ratio exhibited linear and quadratic increasing trends (p < 0.05). The supplementation of 5-HMF at 30,000 mg/kg DM reduced propionate and isobutyrate concentrations under both dietary conditions (p < 0.05). No differences were observed in the alpha diversity indices of the bacterial community among different 5-HMF levels or substrates (p > 0.05). PCoA and PERMANOVA revealed that community structure differed between the 7500 mg/kg DM group and the 0 mg/kg DM group in the T1 diet (p = 0.011), whereas no difference was detected among T2 treatments. LEfSe revealed that 6 and 13 differential bacterial taxa were identified in T1 and T2 at the 5-HMF level of 7500 mg/kg DM, respectively. Correlation analysis revealed that, in T1, GP72 was positively correlated with Pseudomonadota, and the A/P ratio was positively correlated with fibrolytic taxa including Rikenellaceae_RC9_gut_group and Christensenellaceae_R-7_group. In T2, c was positively correlated with Fibrobacterota and Fibrobacter, while NH3-N was positively correlated with Desulfovibrio, indicating diet-specific microbial associations with rumen fermentation (p < 0.05). In conclusion, the high-concentrate diet exhibited stronger rumen fermentation activity. 5-HMF exerted diet-specific effects: In the low-concentrate diet, 5-HMF at 7500 mg/kg DM enriched fibrolytic microbes and shifted fermentation toward an acetate-type pattern. In the high-concentrate diet, it modulated hydrogen-metabolizing bacteria while selectively enriching Pseudomonadota capable of participating in carbohydrate metabolism. The 30,000 mg/kg DM level inhibited rumen fermentation. The findings provide a theoretical reference for the application of 5-HMF in ruminant feeds. However, further in vivo trials are required to validate its effects, and future studies with refined dose gradients based on realistic 5-HMF concentrations in feedstuffs are needed to comprehensively evaluate its efficacy and safety.