The nutritional effects of tannins and related polyphenols in bird resistant and non-bird resistant sorghum varieties and in legume browses
Large amounts of cereal straws are produced in Africa every year. The straw is a potentially valuable source of energy for ruminant livestock. The quality of some straws, notably bird resistant sorghum varieties, are very low (Reed et al . 1987) and may need treatment prior to feeding to the animals. Since physical treatments are often expensive and technically unsuitable for small farming conditions, proven chemical methods could be employed wherever they are technically and economically feasible. Supplementation for deficient nutrients is an alternative approach to improve the utilisation of the straws. Energy and N contents in legume fodder crops and browses make them attractive alternatives to expensive concentrates as supplements for poor quality straws. But often the intake and digestibility of these legumes including browses are reduced by the contents of polyphenolic compounds including tannins (Van Hoven, 1984). These compounds may also reduce digestibility of sorghum stover (Reed et al. 1987). Despite the adverse effects on intake and digestibility tannins protect proteins from degradation by rumen microorganisms and improve the quality of amino acids absorbed in the postrumen gut (Waghorn et al., 1987). They also promote metabolism of endogenous N in the digestive tract (Barry and Manley, 1984). At certain levels of tannins (&'60 10&'37) favourable effects on animal performance can be realised (Ehohe et al, 1981). However it has been noted that large faecal N excretions in ruminants are invariably associated with consumption of high amounts of tannins in the diet. Noting the complexity of the polyphenol-protein interactions it is anticipated that some of the protected proteins may not be available to the digestive enzymes in the postrumen gut as it is often anticipated. The first experiment was therefore concerned with polyphenols in bird resistant and non-bird resistant sorghum leaves and stems; and with their relationship with cell wall (NDF), NDF digestibility and digestible organic matter concentration, before and after alkali treatment. Out of all the alkali treatments conventionally used to improve digestibility in straws urea was not anticipated to be an effective treatment for sorghum stovers that contain high levels of tannins and related polyphenols. These compounds may inhibit urease activity and reduce the production and release of ammonia from urea (Lohan et al., 1981).