Chemical and biological assays for quantification of major plant secondary metabolites
SummaryDuring grazing, animals interact with many compounds in the diet. Plant secondary metabolites (PSM) such as tannins, saponins, cyanogenic glycosides, mimosine. could have wide ranging effects –beneficial or harmful, depending on the nature, quantity in the plant, and amount and rate of ingestion of the PSM. A wide range of methods is available to quantify these PSM. The methods presented for tannins are based on the oxidation-reduction, metal complexing, polyethylene glycol binding and protein precipitation/binding principles of tannins. A tannin bioassay based onin vitrogas production using a medium containing rumen microbes, and incubation of the plant sample with and without polyethylene glycol (a tannin-inactivating agent) is also discussed. Saponins have haemolitic activity, and a qualitative and a quantitative assay are based on this property are described, in addition to a spectrophotometric assay. Cyanogenic glycosides are measured using picric acid methods with and without distillation. The methods described for mimosine determination are based on its reaction with ferric chloride and diazotised p-nitroaniline. Alkaloids are a chemically heterogeneous group of compounds. Therefore, it is difficult to assay alkaloids using a single method. An approach, for qualitative assessment of alkaloids in feeds and forages, based on the use of various spray reagents on the developed TLC plate or a paper chromatographic paper is presented.