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Volume 68, Issue 1 p. 98-107
Article

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

C. T. Robbins

C. T. Robbins

Program in Wildlife Biology, Washington State University, Pullman, Washington 99164-4220 USA

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T. A. Hanley

T. A. Hanley

United States Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station, Forestry Sciences Laboratory, P.O. Box 909, Juneau, Alaska 99802 USA

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A. E. Hagerman

A. E. Hagerman

Department of Chemistry, Miami University, Oxford, Ohio 45056 USA

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O. Hjeljord

O. Hjeljord

Institute of Nature Conservation, Agricultural University of Norway, P.O. Box 39, N- 1432 As-NLH, Norway

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D. L. Baker

D. L. Baker

Colorado Division of Wildlife, 317 West Prospect, Fort Collins, Colorado 80526 USA

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C. C. Schwartz

C. C. Schwartz

Alaska Department of Fish and Game, P.O. Box 3150, Soldotna, Alaska 99669 USA

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W. W. Mautz

W. W. Mautz

Institute of Natural and Environmental Resources, University of New Hampshire, Durham, New Hampshire 03824 USA

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First published: 01 February 1987
Citations: 426

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.