Polyphenols and agriculture: beneficial effects of proanthocyanidins in forages

Proanthocyanidins (PA), also known as condensed tannins, belong to the olde st of plant secondary metabolites. These compounds are widespread in woody plants, but are also found in certain forages. Proanthocyanidins can exert effects in organisms because of their ability to complex with proteins. For ages containing moderate concentrations of PA (2-4% DM) can exert beneficia l effects on protein metabolism in sheep, slowing degradation of dietary pr otein to ammonia by rumen micro-organisms and increasing protein outflow fr om the rumen, thus increasing absorption of amino acids in the small intest ine of the animal. This was shown to result in increases in lactation, wool growth and liveweight gain, without changing voluntary feed intake. Dietar y PA can also contribute to improved animal health by reducing the detrimen tal effects of internal parasites in sheep and the risk of bloat in cattle. In contrast, high dietary PA concentrations (6-12% DM) depress voluntary f eed intake, digestive efficiency and animal productivity. Temperate forages containing PA, such as Lotus corniculatus and L. peduncul atus, will not persist in intensive grazing systems if continuously grazed (i,e. set stocked), especially in mixtures with perennial ryegrass and whit e clover and need to be grown as pure species and rotationally grazed. Neve rtheless, inputs of these 'special purpose 'forages can increase sustainabi lity and productivity in intensive grazing systems through increasing the e fficiency of animal production, reducing urinary nitrogen (N) excretion and reducing chemical inputs as anthelmintics and as detergents used to contro l rumen bloat in cattle. Proanthocyanidins are derived from the flavonoid biosynthesis pathway, and knowledge is rapidly increasing about the molecular control of PA biosynthe sis, These recent investigations may ultimately enable the expression by ge netic engineering of increased levels of PA in the leaves of agriculturally important forage plants such as white clover and perennial rye grass, whic h will withstand continuous defoliation under grazing and currently contain only trace amounts of PA. This could potentially lead to the widespread us e of PA in temperate grazing systems. Further consideration, should now be given to long-term adaptations in the animal, and to ecological effects on the soil ecosystem by PA and their degradation products excreted in animal faeces. More information is required in particular on the effects of PA on soil nitrification, ammonia volatilisation from soil, and nitrogen levels i n the groundwater. (C) 1999 Elsevier Science B.V. All rights reserved.