Effects of Gallnut Tannins and Grape Seed Proanthocyanidins on Alfalfa Silage Quality: Ensiling Characteristics, Protein Degradation, and In Vitro Ruminal Fermentation

This study investigated the effects of gallnut tannins (GT, hydrolysable tannin) and grape seed proanthocyanidins (GSP, condensed tannin), applied alone or in combination, on fermentation quality, protein preservation, and in vitro ruminal performance of alfalfa silage. Alfalfa was ensiled without additives (Con), or with 3% GT (GT3), 2% GT + 1% GSP (GT2P1), 1% GT + 2% GSP (GT1P2), and 3% GSP (GSP3) on a forage dry matter (DM) basis. The results showed that tannin treatments reduced dry matter loss (DML) and non-protein nitrogen contents, increased lactic acid-to-acetic acid ratios, and shifted CNCPS protein fractions toward true protein. Among all treatments, GT2P1 treatment showed the most prominent effect on reducing DML. After 48 h of in vitro incubation, GSP3 treatment resulted in greater reductions in DM and crude protein degradation, and protease activity compared with GT3, GT2P1, and GT1P2 treatments. GT2P1 treatment was more effective than GT3 and GSP3 treatments in increasing propionic acid molar proportion, microbial crude protein synthesis and Fibrobacter succinogenes abundance. These findings demonstrated that ensiling with GT2P1 can improve the ensiling characteristics, protein quality, and in vitro rumen fermentation of alfalfa silage.