Influence of Condensed and Hydrolysable Tannins on the Bacterial Community, Protein Degradation, and Fermentation Quality of Alfalfa Silage

SIMPLE SUMMARY: Proteolysis widely exists in alfalfa ensiling, and up to 88% of total nitrogen could be converted to non-protein nitrogen, leading to low protein utilization efficiency. In order to limit proteolysis, tannins have been used to protect protein from microbial degradation. Although there have been studies that evaluated the efficiency of hydrolysable tannin and condensed tannin to prevent proteolysis, the two were mostly considered individually. In addition, the effects of different types of tannins on plant proteases are unknown. We turned our attention to evaluating the effects of hydrolysable tannin and condensed tannin on the bacterial community and proteolysis of alfalfa silage. In this study, both hydrolysable tannin and condensed tannin were able to reduce proteolysis in alfalfa silages—especially for condensed tannin. The application of hydrolysable tannin decreased the abundance of Lactobacillus and increased the abundances of Enterococcus, while the opposite results were observed when condensed tannin was applied. In addition, condensed tannin reduced the activity of carboxypeptidases and aminopeptidases in treated silage, whereas silage treated with hydrolysable tannin showed lower acid protease and carboxypeptidases activities. Thus, both hydrolysable tannin and condensed tannin could limit proteolysis in alfalfa silage, but the mechanism and their effects on the bacterial communities were different. ABSTRACT: This study evaluated the effects of hydrolysable tannin (HT) and condensed tannin (CT) on the bacterial community, fermentation quality, and proteolysis of alfalfa silage. Alfalfa was wilted to a dry matter (DM) of 35% fresh weight and ensiled with or without 4% HT or 4% CT. The application rates of tannins were based on fresh weight, and each treatment was ensiled in triplicate. After 60 d of fermentation, the CT-treated group had lower concentrations of ammonia nitrogen (NH(3)-N) and free amino acid nitrogen (AA-N), but greater lactic acid concentration, than those in the control and HT-treated silage (p < 0.05). Compared to the control group, the application of tannins increased the abundance of Pseudomonas (negatively correlated with aminopeptidases activity), and decreased the abundance of Pediococcus—which was positively correlated with aminopeptidases activity—and the concentrations of non-protein nitrogen (NPN), NH(3)-N, and AA-N. The application of HT decreased the abundance of Lactobacillus and increased the abundances of Enterococcus, while the opposite results were observed in the CT-treated group. The application of HT and CT reduced the proteolysis in treated silages, but the two were different in terms of their mechanism and their effects on the bacterial communities of the alfalfa silage.