Seasonal variation of phenols, crude protein and cell wall content of birch (Betula pendula Roth.) in relation to ruminant in vitro digestibility.

Birch twigs of diameter ≦1.5 mm exhibit seasonal trends in ruminant in vitro organic matter digestibility (IVOMD), and in the contents of crude protein, cell walls (neutral detergent fibre, NDF), and phenolic compounds. The IVOMD is low in winter twigs, increases in spring, and reaches a maximum in early summer. Crude protein behaves similarly. On the other hand, the proportion of hydrophilic phenols and cell walls (cellulose, hemicellulose, and lignin) to dry weight decreases dramatically in spring when leaves start emerging and growth is initiated. This reduction of phenols is reflected by concomitant changes in concentration of catechin, a major phenolic compound of birch. The concentration of phenolic acids are low in winter and spring but increase after leafing.The biological activity of an extract containing the phenolic compounds, measured as reduction of IVOMD, also decreases concomitantly with the decline of the total phenolic concentration and catechin. It is notable that catechin when tested alone at natural concentrations does not depress IVOMD. It is possible, however, that the amount of catechin reflects the level of condensed tannins, which may be responsible for IVOMD depression. The results strongly indicate that the decline of NDF and phenolic constituents is important for an improved food quality. Phenols may constitute the major chemical defense of birch in winter against browsing vertebrates by reducing digestibility and having toxic properties.