In vitro rumen apparent and true digestibilities of tannin-rich forages

Abstract

Presence of condensed tannins could interfere in the determination of in vitro rumen true digestibility. The neutral detergent fibre (NDF) and acid detergent fibre (ADF) values obtained after removal, by centrifugation, of the liquid phase of the incubation medium containing tannins (centrifuged set) were compared with those obtained directly (direct set) for four tannin-rich leaves ( Dichostachys cinerea, Cassia sieberiana, Robinia pseudoacacia and Acioa barteri). The NDF values in the centrifuged set were significantly higher than in the direct set. On the other hand, ADF values were significantly higher in the direct set. The contents of both nitrogen and condensed tannins were significantly higher in the NDF of the centrifuged set compared with in the direct set, while nitrogen and condensed tannins were significantly higher in the ADF of the direct set. Nitrogen and condensed tannins were present in the ADF of both centrifuged and direct sets at levels higher than in the ADF of the sample incubated. This was similar for the case of NDF of the centrifuged set. These observations suggest that ADF values obtained in the centrifuged or direct set and NDF values in the centrifuged set are incorrect. Following fermentation, the centrifugation process for recovering residue, comprising of unfermented residue and the microbes used for determination of apparent digestibility, and then removal of microbes from this residue by neutral detergent solution treatment for determination of true digestibility lead to serious artefacts in these digestibility values. Chromatographic assays based on measurement of free monomeric constituents are recommended for determination of fibre in tannin-rich feeds.