Models of suppressive effect of tannins. Analysis of the suppressive effect of tannins on ruminal degradation by compartmental models

Abstract

Three compartmental models were developed to evaluate the suppressive effects of tannins on the extent and rate of degradation of tree foliage. The first model was based on the assumption that tannins modify the parameters of the degradation kinetics, the second was based on the assumption that tannins bind material, independent of the basic degradation kinetics, and the third combined these two assumptions, i.e., that tannins both suppress degradation and bind free material. Degradation was measured by suspending the samples in dacron bags in the rumens of goats, with or without the inclusion of polyethylene glycol (PEG). It was assumed that PEG annuls the suppressive effect of tannins on degradation of plant components, and hence that the difference between the curves that describe degradation with or without PEG indicates the suppressive effect of tannins present in the foliage. The data on the observed degradation of dry matter, neutral detergent fibre and protein, in four typical Mediterranean forest tree species were fitted by these models. It was found that the combined model fits the data better than either of the other two models. Increasing content of tannins in the foliage was associated with an increase of the bound free material and a decrease in the degradation rate of the degradable matter, but not with an increase of the non-degradable fraction.

Introduction

Trees and shrubs are important feeds for ruminants in dry environments, but their use is often limited by phenolic compounds such as tannins, which can adversely affect digestibility and intake (Kumar and Vaithiyanathan, 1990; Silanikove et al., 1994). The major negative effect of these compounds results from either formation of the indigestible compounds with food and endogenous protein, or direct inhibition of digestive enzymes (Kumar and Vaithiyanathan, 1990). Inclusion of polyethylene glycol (PEG) with a molecular weight of 4000 in tannin-containing leaf diets for sheep (Pritchard et al., 1992; Terril et al., 1992; Silanikove et al., 1994) improved digestibility, dry matter (DM) intake, and performance. In vitro studies showed that the addition of PEG to plant samples containing tannins, increased the digestibility of DM and protein, (Garrido et al., 1991). This effect of PEG on tannin-containing diets arises because tannins form complexes with PEG more readily than with proteins, and these complexes are stable under the chemical conditions prevailing in the digestive tract (Jones and Mangan, 1977). Recently, an assay of tannin content and their negative effect on ruminal degradation was developed, using the tannins binding capacity of PEG (Silanikove et al., 1996).

To evaluate the suppressive effect of tannins on degradability, we assumed that when 2 g PEG is added to each 5-g sample of dried and ground foliage, the suppressive effect of tannins is completely annulled and hence that the net effect of tannins could be indicated by the difference between samples that were incubated with or without PEG. The effect on the effective degradability (E, Ørskov and McDonald, 1979) could be evaluated by the difference between the two values of E, obtained in the presence and absence of PEG.

The goals of the present study were: (i) to test the ability of models of the degradation kinetics to fit data of tannin-containing material that disappeared during its incubation for up to 72 h in dacron bags, with or without the presence of PEG, in the rumens of goats, and (ii) to relate the kinetic parameters estimated by these models to the tannin content of the foliage, and to the extent of suppression of degradability.