Silanikove, N., Department of Cattle Physiology and Nutrition, Institute of Animal Science, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50 250, Israel
Perevolotsky, A., Department of Natural Resources, Institute of Field and Garden Crops, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50 250, Israel
Provenza, F.D., Range Science Department, Utah State University, Logan, UT 84322, United States

Synthetic polymers such as water-soluble polyvinyl pyrrolidone (PVP), water-insoluble polyvinyl polypyrrolidone (PVPP), and water-soluble polyethylene glycol (PEG) contain sufficient oxygen molecules in a chain to form strong hydrogen bonds with the phenolic and hydroxyl groups in tannins. This review deals with the practical uses of tannin-binding agents, particularly PEG, in tannins assays and for determining the negative effects of tannins on feed intake and digestion in ruminants. A gravimetric method to assay tannins by precipitation with PVPP is specific for tannins and does not require standards. The extractability of tannins from plant tissues can be reduced by drying samples at temperatures above 50°C and is dependent on many other factors, such as content and types of plant proteins. Therefore, it is not feasible to recommend a single, optimal protocol for all plant samples. A method to assay tannins which is based on measuring the amount of binding of PEG to plant samples was shown to be simple and accurate. It can also overcome some of these extractability problems. The following biological effects of tannins were investigated in studies where tannin anti-nutritional effects were partially or completely neutralised by varying levels of PEG supplementation. (1) Effects on appetite: the negative effects of tannins on appetite can occur in the short-term (within 20-60 min) and the long-term (days and weeks). Astringency and adverse postingestive influences of tannins on the epithelium of the oral cavity and the foregut cause short-term effects on food intake. Long-term effects can be related to reduction in the concentration of ammonia and volatile fatty acids (VFA) in rumen fluids, which can in turn serve as metabolic cues for deficiency of nitrogen (ammonia), energy (VFA), or both. (2) Effects on digestion: increasing content of tannins in foliage can be associated with an increase in bound protein and with reduced degradation rate of the degradable matter in the rumen, but there is no corresponding increase of the non-degradable fraction. Consequently, organic matter, protein, and cell wall digestibility are inversely related to tannin concentrations. (3) Inducing digestive responses: if a significant amount of tannins reach the duodenum, they may markedly reduce the intestinal activity of pancreatic enzymes (trypsin and amylase) and amino acids absorption from the intestine. Condensed tannins can also reduce the content of fluid and particulate matter in the rumen, accelerate the passage of liquid from the abomasum, and delay the passage of digesta in the intestine. The overall effect is a delay in the passage of fluid and particulate matter throughout the entire gastrointestinal tract. It is hypothesised that these responses are largely the consequence of the interaction of tannins with digestive enzymes and the epithelium lining the digestive tract. © 2001 Elsevier Science B.V.