Survey of the Variation in Grape Marc Condensed Tannin Composition and Concentration and Analysis of Key Compositional Factors

Grape marc contains a number of compounds with potential antimethanogenic activity in ruminants, including condensed tannins (CTs). Using direct phloroglucinolysis, a survey of CT chemistry across 66 grape marc samples showed diversity in concentration (6.9 to 138.8 g/kg of dry matter). Concentration was found to be independent of CT composition, although all compositional variables were significantly correlated (P < 0.0001). Twenty samples diverse in CT were selected from a cluster analysis and analyzed for compounds relevant to ruminant digestion and methanogenesis, including metabolizable energy (6.6–12.0 MJ/kg DM), crude protein (3.2–14.4% DM), neutral detergent fiber (18.4–61.4% DM), and ethanol soluble carbohydrates (2.0–40.6% DM). Fatty acid concentrations varied throughout the 20 samples (5.2–184.5 g/kg DM), although fatty acid profile showed two distinct groups. Grape marc varies widely in nutritional value, and in compounds that have been linked with changes in ruminant digestion and methane emissions.