Binding of Poly(ethylene glycol) to Samples of Forage Plants as an Assay of Tannins and Their Negative Effects on Ruminal Degradation

A procedure to measure poly(ethylene glycol) (PEG) binding to plant samples is described. The amount of PEG bound to a sample was found to be reproducible with a small coefficient of variation (0.5%). The PEG-binding capacity of plant samples was not affected by predrying of the samples at 90 °C, unlike the colorimetric methods for determination of tannins. Both the PEG-binding and the protein precipitation capacity of the plant samples were found to be useful in predicting the negative effects of tannins on ruminal degradation of the plant material. However, PEG binding to plant samples had an advantage over protein precipitation in samples in which formation of strong tannin−protein complexes results in low extractability of tannins. For this reason, the binding method was found to be preferable to the protein precipitation method in predicting the negative effect of tannins on ruminal degradation of forage plants.