Comparison between analytical methods and biological assays for the assessment of tannin-related antinutritive effects in some Spanish browse species

Twenty-two samples of leaves and flowers of six Mediterranean browse species (Erica australis, Quercus pyrenaica, Cistus laurifolius, Cytisus scoparius, Genista florida and Rosa canina) were analysed for total extractable phenols (TP), extractable tannins (ET) and condensed tannins (CTb (butanol/HCl assay) and CTv (vanillin assay)). The biological activity of the phenolic compounds was assessed as the increases in in vitro dry matter digestibility and gas production after in vitro incubation in rumen fluid following the addition of polyethylene glycol 6000 (PEG). The concentrations of phenolics and tannins varied with species, sampling time and part of the plant (leaves or flowers). There were also large differences in the values determined by different analytical methods. The highest correlation coefficients (P < 0.001) were found between TP and ET (r = 0.973) and between CTb and CTv (r = 0.758). However, only low correlations were observed between TP and CTb or CTv. Positive and significant (P < 0.01) correlations were observed between tannins and their biological activity determined as the effect of PEG on gas production. It is concluded that, although further research is required to develop methods for the accurate quantification of biologically active tannins, the effect of PEG on in vitro gas production seems to have promising potential for the assessment of phenolic-related antinutritive effects in browse plants. This technique, coupled with other tannin assays, could provide a better understanding of the nutritional and physiological significance of tannins.