A review of the effects of forage condensed tannins on ruminal fermentation and bloat in grazing cattle

Condensed tannins (CT) are polyphenolic secondary plant products that are w idespread in the plant kingdom. By definition, CT bind to protein and are r egarded as "antinutritional" compounds which reduce protein digestibility. Variations in CT chemistry alter protein binding capacities among polymers from different plant species and developmental stages. Condensed tannins in teract with proteins in feed, saliva and microbial cells, with microbial ex oenzymes, and with endogenous proteins or other feed components, which alte rs digestive processes as compared with diets free of CT. Tannin levels exc eeding 40 to 50 g kg(-1) DM in forages may reduce protein and DM digestibil ity of the forages by ruminants. At low to moderate levels, CT increase the quantity of dietary protein, especially essential amino acids, flowing to the small intestine. Unlike alfalfa, legumes that contain CT do not cause b loat. Dietary CT may provide a means to beneficially manipulate protein dig estion and/or prevent pasture bloat in ruminants. Research efforts are also currently being directed to genetically modify alfalfa to de-repress its C T biosynthetic pathway, or to move genes encoding steps of this pathway int o alfalfa from other legumes. The effects of CT in forage legumes on digest ion in ruminants and the potential use of tannin-rich forages for preventin g bloat in grazing systems are discussed.