Role of Tannins in Defending Plants Against Ruminants: Reduction in Dry Matter Digestion?

Polyphenolic allelochemicals, such as tannins, are widely thought to reduce the digestibility of plants consumed by herbivores by binding to digestive enzymes and dietary proteins. While the apparent digestibility of protein and, therefore, cell solubles is reduced in mule deer (Odocoileus hemionus) and white-tailed deer (O. virginianus) consuming tanniferous forages, digestion of the plant cell wall is not reduced beyond that predicted from its content of lignin, cutin, and silica. The lack of a tannin effect on cell wall digestion in deer is in contrast to studies with domestic sheep and numerous in vitro studies. Herbivores adapted to consume tanniferous forages may defend against such allelochemicals by producing salivary proteins that bind tannins in a highly specific manner. These tannin-salivary protein complexes would reduce apparent digestibilities of protein and cell solubles and, if completely effective, would not reduce cell wall digestion. The occurrence of such proteins in ruminants is reported here for the first time. The saliva composition of mule deer (a mixed feeder that commonly consumes browse) and domestic cattle and sheep (predominant grazers) are compared, and the higher potential of the deer saliva to neutralize tannins is related to their feeding habits. Salivary proteins that preferentially bind tannins may minimize fecal nitrogen losses by maximizing the efficiency of tannin-binding per unit of protein and may reduce the absorption of hydrolyzable tannins and the potential for tannin toxicity.