Role of Tannins in Defending Plants Against Ruminants: Reduction in Protein Availability

Abstract

We tested the hypothesis that tannins defend plants against large herbivores by decreasing protein availability. Digestion trials were conducted with mule deer (Odocoileus hemionus) and results from previous trials with white—tailed deer (O. virginianus), moose (Alces alces), caribou/reindeer (Rangifer tarandus), and elk (Cervus elaphus) were summarized to evaluate dietary factors affecting protein availability. The digestibility of plant protein in feeds with minimal tannins, such as grasses and agriculturally produced legumes and grains, was a highly predictable function of the total protein content and the amount of nondigestible, fiber—bound protein. Digestible protein in plants containing significant tannins was lower than predicted from regressions for low—tannin feeds. The reduction in digestible protein was proportional to the protein—precipitating capacity of the plant tannins. Deciduous browse stems collected in winter had very low levels of protein—precipitating tannins and only a slightly lower protein availability than predicted. Tannins are not important in the defense of most deciduous tree shrub stems consumed by these herbivores. Tannins in flowers and forb, tree, and shrub leaves markedly reduced protein availability. Tannins must be considered in understanding the defensive strategies of leaves and flowers. Voluntary intake of the high—phenolic forages was significantly reduced below ingestion rates for grasses, legumes, and pelleted diets. It is hypothesized that soluble phenolics that do not inhibit digestion but are absorbed and reduced intake through their toxicity are more important in defending some plant parts against ruminants than are digestion—reducing tannins.