N. Silanikove et al., Use of tannin-binding chemicals to assay for tannins and their negative postingestive effects in ruminants, ANIM FEED S, 91(1-2), 2001, pp. 69-81
Synthetic polymers such as water-soluble polyvinyl pyrrolidone (PVP), water
-insoluble polyvinyl polypyrrolidone (PVPP), and water-soluble polyethylene
glycol (PEG) contain sufficient oxygen molecules in a chain to form strong
hydrogen bonds with the phenolic and hydroxyl groups in tannins. This revi
ew deals with the practical uses of tannin-binding agents, particularly PEG
, in tannins assays and for determining the negative effects of tannins on
feed intake and digestion in ruminants. A gravimetric method to assay tanni
ns by precipitation with PVPP is specific for tannins and does not require
standards. The extractability of tannins from plant tissues can be reduced
by drying samples at temperatures above 50 degreesC and is dependent on man
y other factors, such as content and types of plant proteins. Therefore, it
is not feasible to recommend a single, optimal protocol for all plant samp
les. A method to assay tannins which is based on measuring the amount of bi
nding of PEG to plant samples was shown to be simple and accurate. It can a
lso overcome some of these extractability problems. The following biologica
l effects of tannins were investigated in studies where tannin anti-nutriti
onal effects were partially or completely neutralised by varying levels of
PEG supplementation. (1) Effects on appetite: the negative effects of tanni
ns on appetite can occur in the short-term (within 20-60 min) and the long-
term (days and weeks). Astringency and adverse postingestive influences of
tannins on the epithelium of the oral cavity and the foregut cause shortter
m effects on food intake. Long-term effects can be related to reduction in
the concentration of ammonia and volatile fatty acids (VFA) in rumen fluids
, which can in turn serve as metabolic cues for deficiency of nitrogen (amm
onia), energy (VFA), or both. (2) Effects on digestion: increasing content
of tannins in foliage can be associated with an increase in bound protein a
nd with reduced degradation rate of the degradable matter in the rumen, but
there is no corresponding increase of the non-degradable fraction. Consequ
ently, organic matter, protein, and cell wall digestibility are inversely r
elated to tannin concentrations. (3) Inducing digestive responses: if a sig
nificant amount of tannins reach the duodenum, they may markedly reduce the
intestinal activity of pancreatic enzymes (trypsin and amylase) and amino
acids absorption from the intestine. Condensed tannins can also reduce the
content of fluid and particulate matter in the rumen, accelerate the passag
e of Liquid from the abomasum, and delay the passage of digesta in the inte
stine. The overall effect is a delay in the passage of fluid and particulat
e matter throughout the entire gastrointestinal tract. It is hypothesised t
hat these responses are largely the consequence of the interaction of tanni
ns with digestive enzymes and the epithelium lining the digestive tract. (C
) 2001 Elsevier Science B.V. All rights reserved.