CHANGING PERCEPTIONS OF THE EFFECT OF PLANT PHENOLICS ON NUTRIENT SUPPLY IN THE RUMINANT

Mammalian metabolism of plant phenolics, initially studied in monogast ric animals, gave an emphasis to their toxic and antinutrient effects. Subsequent studies in tropical ruminants and wild herbivores have hig hlighted the high levels than can occur in some diets and tile extensi ve microbial modification and degradation that can occur in the tract. This paper reviews aspects of plant phenolics as they relate to rumin ant nutrition in tropical or semi-arid environments in which some fora ge plants contain high levels of phenolic compounds. Effects range fro m occasional acute toxicity of hydrolysable tannins, to acetate-releas ing microbial degradations that apparently enable certain phenolics to act as nutrients. The most important and complex effects are those du e to tannin-protein interactions. Although these can clearly reduce fe ed intake, nutrient digestibilities, and protein availability, many of the interactions are still not understood. The diverse effects of pla nt phenolics on nutrient flow probably result from the balance between adverse effects on some organisms and the rate at which they are degr aded or inactivated by other organisms, and improved animal performanc e can likely be obtained by manipulation of rumen microbial metabolism .