Relationships among Soluble Phenolics, Insoluble Proanthocyanidins and Fiber in East African Browse Species

Abstract

Tannins and other phenolics interfere with the interpretation of results from the detergent system of forage analysis. Leaves and apices from browse can contain up to 50% of their organic matter as phenolics including tannins that are soluble in aqueous acetone. Leaves and apices from browse that contain soluble proanthocyanidins (condensed tannins) also contain proanthocyanidins that are insoluble in both aqueous acetone and neutral-detergent. The content of insoluble proanthocyanidins is positively correlated with neutral-detergent fiber (NDF) and fiber-bound nitrogen. Condensed tannins may bind protein and make it less soluble in neutral-detergent and increase the content of NDF. The behavior of phenolics and tannins in the detergent system of forage analysis is discussed in relationship to estimating the nutritive value of 17 East African browse species. The detergent system of forage fiber analysis and the partitioning of cell wall from cell contents is widely used in determining the nutritive value of browse in the diets of mammalian herbivores (Van Soest 1967, 1982; Mould and Robbins 1981). Soluble phenolics and tannins can lower nutritive value of forages by toxic effects on the animal or through enzyme inhibition and substrate binding in the digestive tract (Wong 1973, Swain 1979). Insoluble proanthocyanidins (condensed tannins) occur in the leaves of legumes and other plant species (Bate-Smith 1973, 1975), but their effects on nutritive values have not been sufficiently studied. Insoluble proanthocyanidins may be involved in plant defense, associated with insoluble and enzyme resistant protein, and negatively associated with digestibility and palatability (Ford 1978, Stafford and Cheng 1980, Reed et al. 1982). An understanding of the relationship of tannins and other phenolics to the detergent system is essential to its use in predicting nutritive value of browse (Mould and Robbins 198 1). Materials and Methods Browse samples were collected over a 9-month period, including I dry season and 2 wet seasons, in Kenya during a study on the nutritional ecology of game and cattle (Reed 1983). Leaves and apices from 17 species, 39 samples (Table 1), were air dried in a well-ventilated room. Due to low humidity drying occurred in 24 to 48 hours. The species sampled are important in the diets of wild and domestic ungulates. Neutral-detergent fiber (NDF), acid-detergent fiber (ADF), acid-detergent lignin, hemicellulose, and cellulose were determined by the methods of Goering and Van Soest (1970), omitting decahydronaphthalene and sodium sulfite in the NDF procedure (Van Soest and Robertson 1980). Proanthocyanidins insoluble in neutral detergent were determined by methods described by BateSmith (1973) as modified by Reed et al. (1982). Five milligrams of NDF were placed in a test tube, 5 ml of 5% concentrated HCI in a n-butanol were added and the tubes heated at 100?C for I hour. Absorbance was read at 550 nm. If proanthocyanidins were present in the NDF, the n-butanol-HCl solution turned red when heated. Results were expressed as A550 per gram of NDF. Nitrogen in the NDF (NDF-N) of Acacia species was determined by a macro-Kjeldahl procedure using Na2SO4 and CUS04 in the digesAuthor is animal nutritionist, International Livestock Centre for Africa, Addis Ababa, Ethiopia, P.O. Box 5689. Manuscript accepted 9 May 1985. Table 1. Neutral-detergent fiber (NDF), soluble phenolics and insoluble proanthocyanidins in East African browse.