Tannins in faba beans (Vicia faba L.) : antinutritional properties in monogastric animals

Condensed tannins are found in coloured-flowering varieties of faba beans ( Viciafaba L.). They are considered as antinutritional factors for nonruminant species. High-tannin hulls of faba beans and isolated tannins were shown to induce a rapid hypertrophy of the parotid glands in rats and increase the secretion of salivary proline-rich proteins with a high affinity for tannins. In this way rats are able to defend themselves against more harmful effects of tannins. The growth performance of chickens on diets containing 300 g/kg of either low-tannin or hightannin faba beans did not differ. In contrast, the apparent ileal and faecal digestibility in pigs of protein and amino acids differed significantly between both types of beans, in favour of the low-tannin beans. Inclusion of hulls of faba beans with either a low or a high tannin content in diets for piglets revealed that tannins in faba beans reduce in particular the apparent faecal digestibility of protein and amino acids and the N balance. No systemic/toxic effects of feeding high-tannin hulls to pigs were found. The activity of trypsin and, to a lesser extent, chymotrypsin was reduced in digesta collected from the small intestine of pigs. It was shown with the ¹⁵N isotope dilution technique that about half of the extra protein appearing in ileal digesta and faeces of pigs when feeding faba bean hulls with a high tannin content is of endogenous origin. The other half consists of dietary protein. Tannins from faba beans show some preference to interact with proteins with a high content of proline and histidine. A concept for the effects and mode of action of faba bean tannins in different monogastric animal species is discussed.