The use of hydrolysable tannins (HTs) and condensed tannins (CTs) at different mixing ratios and dosages can affect rumen fermentation. Therefore, the binding capacity of mixed tannins to protein and cellulose in feed and their mechanisms, are of critical importance. In this experiment, HTs and CTs were added to the diet at concentrations of 1%, 2%, and 3% dry matter (DM) in combinations of HTs alone, CTs alone, and ratios of (HT:CT) 3:7, 4:6, 1:1, 6:4, and 7:3, respectively. In situ degradation and in vitro fermentation experiments were then conducted. In both the in situ degradation tests and the in vitro rumen fermentation experiments, the effects of CTs and the 4:6 treatment group on the degradation rates of dry matter (DM) and neutral detergent fiber (NDF) were consistent. The crude protein (CP), ammonia nitrogen (NH3-N), and acetate levels significantly decreased with increasing proportions of CTs, whereas gas production exhibited the opposite trend. Molecular docking analysis further elucidated the differences in binding capacity between tannins and proteins/cellulose, revealing binding patterns. Gallic acid, the predominant component in HTs, exhibited significantly lower binding affinity to both ligand proteins and cellulose than catechin and epicatechin, the main components in CTs. Regarding the abundances of rumen microorganisms, the abundance of Firmicutes significantly decreased in all the tannin treatment groups, with the most pronounced decreases observed in the 6:4 and 3:7 treatment groups. The 6:4 treatment significantly increased the abundances of RFN20, Prevotella, and Fibrobacter. The 6:4 and 3:7 treatments significantly altered the bacterial genus composition. These findings reveal the strength and mechanisms through which tannins bind to nutrients and confirm the proportional and dose-dependent effects resulting from ruminal fermentation and the presence of tannins at the microbial level. Our study combined HTs and CTs at various ratios and doses, which revealed that the effects of tannins on feed degradation rates and microbial activity in the rumen were not merely additive. Additionally, molecular docking analysis elucidated the binding modes and affinities of the tannins. These findings provide a reference for the rational application of tannins in future studies. The proportion of tannin ratio and the dose‒response effect should be applied to the diets of ruminants. When the ratio of condensed tannins/hydrolysable tannins was 4:6, stronger nutrient-binding properties were observed. The stronger binding capacity of condensed tannins is because catechins and epicatechins adopt more stable binding conformations with protein structures. When the ratio of condensed tannins/hydrolysable tannins was 6:4 or 3:7, the greatest impact on the microbial community in the rumen fluid occurred.
Zhang et al. (Tue,) studied this question.