Phenolics, such as tannins, have been implicated as inhibitors of digestibility. A wide range of assays for measuring phenols have been proposed (reviewed by Hagerman and Butler, 1989) but there is little experimental evidence to indicate which are most appropriate for assessing nutritive value. The objective of this study was to investigate correlations between phenol assays and the effects of phenolics on in vitro fermentation by rumen micro-organisms to identify which assays most reliably indicated the extent of these effects.

Extracts were prepared from 12 Bolivian tree leaf samples using 70% aqueous acetone (500mg dried sample extracted with 5 ml solvent) and analysed for total phenols (TP) by the Prussian Blue method (Price and Butler, 1977), protein precipitation activity (PPA) by a radial diffusion technique (Hagerman, 1987) and condensed tannins (CT) by acid butanol (Porter et al, 1986). The samples used are listed in Table 1.