PREDICTING DIGESTIBLE PROTEIN AND DIGESTIBLE DRY-MATTER IN TANNIN-CONTAINING FORAGES CONSUMED BY RUMINANTS

Equations for predicting digestible protein and digestible dry matter on the basis of laboratory analytical techniques (Robbins et al. 1987 a, b) were tested with in vivo digestion trials using black-tailed deer (Odocoileus hemionus sitkensis). Leaves of seven natural, tannin-containing forages and twigs from one browse species were fed fresh in total collection digestion trials. The equation for estimating digestible protein was based on the plant's crude protein content and a measure of the tannin's protein-precipitating capacity. The equation for estimating digestible dry matter was based on a measure of tannin astringency and the concentrations of neutral detergent fiber, lignin, cutin, and biogenic silica. Both equations predicted in vivo results accurately, although a slight modification of the laboratory technique was recommended for determining neutral detergent fiber. Sodium sulfite should be used when analyzing tannin-containing forb and shrub leaves and omitted for relatively tannin-free forages. The equations provide useful measures of both the ecological effectiveness of tannins as digestion-reducing agents and the nutritional value of the ingested plant matter for the animal.