A new index to estimate reactivity and biological effect of tannins, using tropical browse legumes as a model

The aim of this paper was to propose a new approach to evaluate the effect of tannins on rumen microbial fermentation in terms of biological effect (proportion of the response if tannins are inactivated, BE) and reactivity (capability of tannins to bind chemically with different amounts of a reagent), by recording gas production in 24 h in vitro incubation of substrates with increasing levels (0.025-1 g/g substrate) of polyethylene glycol (PEG). Three tropical browse legumes with different tannin contents (Acacia cornigera, Albizia lebbekoides and Leucaena leucocephala) were chosen as model substrates. The reactivity index (RI, mg PEG) and absolute reactivity index (aRI, mg PEG/ml gas) were estimated from the rate coefficient of fitting gas production per unit of added PEG to an exponential equation for each incubation time. The RI characterises each specific tannin source, and depends on the potential fermentation of the plant. The average RI values of A. cornigera, L. leucocephala and A. lebbekoides were 70.4, 36.3 and 55.7 mg PEG, respectively. The aRI is a tool for species comparison, because it is referred to the total volume of gas produced. It was higher in A. lebbekoides, intermediate in A. cornigera and lowest in L. leucocephala (1.57, 0.76 and 1.14 mg PEG/ml gas, respectively; P=0.012). This index was higher at 6 h than at 12 and 24 h (1.62, 0.96 and 0.89 mg PEG/ml gas, P<0.001). The estimation of these indexes is a simple and inexpensive tool that gives a more complete view of the biological activity pattern of tannins in practical conditions, complementing that of other biological effect indexes and avoiding the biases that arise from their chemical analysis.