Chemical defense in birch: Inhibition of digestibility in ruminants by phenolic extracts

The biological activity of phenolic extracts originating from winter twigs of birch (Betula pendula Roth.) was measured using the ruminant in vitro method and the nylon bag technique. Different extracts were prepared by extraction with organic solvents, removing phenols of corresponding solubility. The extract of birch twigs (diameter <1.5 mm) contained about 19% phenol equivalents, corresponding to 6% of twig dry matter (DM). Coarse birch twigs (diameter 1.5-5 mm) contained about 3% in the DM Phenolic extracts from the fine birch twigs were added to coarse birch twigs and common timothy (Phleum pratense L.) to mimic natural concentrations of fine birch twigs. Controls and the plant material with phenolic extract added were incubated for different times with rumen inocula taken from a sheep fed browse and a goat fed hay. Nylon bags containing phenolic treated hay were incubated in the rumen of the goat for 6 and 48 h. Phenolic extracts had a considerable negative effect on the organic matter (OM), protein and cell wall (neutral detergent fiber, NDF) digestibility in vitro. The nylon bag OM disappearance was also depressed by the extract. The effects were measurable after 6 h of digestion both in vitro and in sacco.The high inhibitory effect by the extracts on digestibility persisted even after removal of lipophilic fractions. This suggests that some or several water-soluble phenolic substances are responsible for the depression of digestibility. The depression of OM digestibility is linearly related to the concentration of phenols added. However, the inhibition of nylon bag digestibility plateaus at high phenol concentrations, suggesting that some fraction of the substances undergo complex formation with macromolecules of the plant.The results strongly indicate that water-soluble phenols of birch make up an important part of its chemical defense in winter by possessing antinutritional properties. Thus their potential importance in the nutrition of wild herbivores must not be ignored.