Chemical, Protein Precipitation and Bioassays for Tannins, Tannin Levels and Activity in Unconventional Feeds, and Efects and Fate of Tannins

Abstract

Animal production systems in the tropical and subtropical countries utilize a wide range of feedstuffs; the main amongst these being the crop and industrial by-products, grasses, legumes, trees and shrubs. Trees and shrubs are of importance in animal production because they can provide significant protein supplements, especially in the dry season. But, the amount of tannins that they contain vary widely and largely unpredictably, and their effects on animals range from beneficial to toxicity and death. Similarly many agro-industrial by-products contain tannins. With a better understanding of tannin properties and proper management, they could become invaluable source of protein for strategic supplementation. As the demand for food rises, these unconventional feedstuffs must play an increasingly important part in the diet of animals, in particular for ruminants in small-holder farming in developing countries. It is therefore critical that proper techniques be used to measure and manage the anti-nutritional effects they cause. The analysis of tannins remains highly problematic. Various chemical assays for hydrolysable tannins and condensed tannins are available. Most tannin-containing feedstuffs contain both hydrolysable tannins and condensed tannins, but unfortunately only condensed tannins are generally analysed, probably because of simplicity of the assays measuring these; and the feedstuffs termed as tannin-containing or tannin-free feedstuffs. Furthermore, the biological effects are ascribed to mostly the condensed tannins (or the synonym: proanthocyanidins), which might be misleading. This paper reviews the available assays for hydrolysable tannins and condensed tannins and highlights the advantages and disadvantages of each. Protein precipitation assays, both isotopic and non-isotopic, representing the operational property of both hydrolysable tannins and condensed tannins; a tannin bioassay based on an in vitro simulation of the rumen and measurement of tannin activity for both free and bound tannins in terms of rumen fermentation parameters; and 14C-polyethylene glycol binding assay are also discussed. Each type of tannin responds differently in each of these assays. This variability makes it impossible to use any single method. Use of a battery of methods, therefore, is suggested; and these assays are being used in the FAO/IAEA-sponsored projects on the utilisation of tree foliage as livestock feed. Using these assays for 37 shrub and tree leaves, highly significant correlation existed between protein precipitation capacity and extractable total phenols (r = 0.87) or tannins (r = 0.83). On the other hand, a weak correlation was observed between condensed tannins (measured by the butanol-HC1 method) and protein precipitation capacity (r = 0.41), which could be due to the variation in structural and biological activity of tannins. The correlations observed between extractable total phenols, tannins or condensed tannins and the tannin bioassay values based on the rumen simulation technique were similar to those obtained between extractable total phenols, tannins or condensed tannins and protein precipitation capacity. Highly significant correlations between extractable phenolics or tannins with protein precipitation capacity or the values obtained using the tannin bioassay suggest that extractable total phenolics and tannins values could be taken as a measure of biological activity of tannins. The condensed tannins values by the butanol-HC1-iron method do not appear to reflect the biological activity. From the relationships between chemical, protein precipitation and bioassays, it was postulated that tree and shrub leaves with extractable total phenol and tannin contents of approximately 4.5% and 2.0% respectively (as tannic acid equivalent) will not produce significant adverse effects on ruminant livestock. The hydrolysable tannins, measured by an HPLC and a spectrophotometric method (rhodanine), were present in all the 37 samples (one having ca 14%; three between 1.5 and 3.5%; six ca 0.5% and the rest below 0.5%) analysed; and the hydrolysable tannins bind proteins, affect rumen fermentation, and could cause adverse effects similar to condensed tannins. Hydrolysable tannins are also known to be toxic and can cause death of animals if consumed in large quantities. The roles of rumen microbes in degradation and overcoming the detrimental effects of tannins, and the effects and fate of tannins in ruminants are also discussed. Simple methods based on postharvest technology, treatment with low-cost chemicals, biological treatments, and supplementation with tannin-complexing agents, to enhance the feeding value of tannin-containing feeds are needed.