Determination of microbial degradability of N is important in
formulating a sound supplementation strategy for efficient utilisation of
basal as well as supplementary diet components. In
vitro degradability of N (IVDN) from tannin-containing
browses (Acacia cyanophylla, Acacia albida, Acioa
barteri and Quercus ilex) and
two herbaceous legumes (Desmodium intortum
andDesmodium uncinatum) was determined using
the in vitro gas-production method coupled with
NH3-N measurement in the presence and absence of a
tannin-binding agent (polyethylene glycol (PEG), molecular mass 6000).
Addition of PEG to tannin-containing feeds significantly
(P < 0·05) increased in
vitro gas and short-chain fatty acid (SCFA) production,
and IVDN. The use of PEG as a tannin-binding agent increased IVDN from 28 to
59, 32 to 72, 19 to 40, 32 to 73, 40 to 80, and 26 to 77 % in
A. cyanophylla, A. albida, A. barteri,
D. intortum,D.
uncinatum and Q.
ilexrespectively. There was significant correlation between
total phenolic compounds (total phenol, TP; total tannin, TT) in leguminous
forages and percentage increase in IVDN on addition of PEG
(P < 0·05;
R2 0·70 and 0·82 for TP
and TT respectively). The difference in IVDN observed in the absence and
presence of PEG indicates the amount of protein protected from degradation
in the rumen by tannins. When measured after 24 h incubation,
tannin-containing feeds incubated in absence of PEG resulted in higher
microbial protein synthesis than in the presence of PEG. Addition of PEG
significantly (P < 0·05) reduced the
efficiency of microbial protein synthesis expressed as μmol purine/mmol
SCFA.