Effect of drying method and assay methodology on detergent fiber analysis in plants containing condensed tannins

Laboratory evaluation of fiber concentration and nutritive value of plants containing condensed tannins (CT) can be complicated by the formation of strong complexes between CT and other molecules as affected by sample preparation and assay methodology. This study looked at fiber and acid detergent insoluble nitrogen (ADIN) concentrations of 10 herbaceous and brushy species with and without naturally occurring CT. Freeze drying (FD) and oven drying (OD) were compared with and without the addition of sodium sulfite (Na2SO3) during sequential neutral detergent fiber assayed with amylase followed by acid detergent fiber and expressed exclusive of ash (aNDF-ADFom) and separate acid detergent fiber expressed exclusive of ash (ADFom-ADF). Results highlight the limitations associated with the detergent fiber analysis for plant material containing CT. Responses varied among the general groups (legume versus non-legumes versus grasses) of plant species studied as well as across species within groups. However, in most cases, reduced values of aNDFom, ADFom and ADIN were observed when plant material was FD rather than OD and when sodium sulfite was added during the neutral detergent assay. Caution should be used when interpreting results obtained from the detergent fiber assay, especially of CT-containing plant material as related to nutrient availability to animals.