Initially the fate of 5 individual phenolic compounds (quercetin, rutin, gallic acid, tannic acid and quebracho tannin) in the rumen was investigated. Using a combination of in vivo and in vitro techniques it was shown that the low molecular weight compounds were rapidly degraded in the rumen and had little effect upon rumen fermentation. Degradation was rapid even when the materials were present in plant material. Tannic acid and to a greater extent quebracho tannin did however escape metabolism in the rumen and reached the duodenum. Quebracho tannin was used as a model in the subsequent studies.
Administration of a single dose (6 g) of quebracho tannin directly into the rumen of 4 rumen-fistulated sheep along with the liquid phase marker, CrEDTA, showed that the average half life of quebracho tannin in the rumen was in the order of 11.0 :f: 3.15 h while that of Cr was 16.25 :f: 5.16 h. Continuous administration of quebracho tannin directly into the rumen of 4 sheep for 7 days confirmed that the material was substantially resistant to microbial attack. When fed to sheep as part of a grassmeal diet, quebracho tannin reduced the digestibility of both the fibre and nitrogenous components of the diet. There was also evidence of focal surface epithelial degeneration in the jejunum and ileum. Damage to the specialised epithelium overlying Peyers patches was also evident and the tannin appeared to be taken up by Peyers patch M cells which, coupled with other changes to the gut wall, could result in a modification of the immune competence of the animal. Other studies indicated that the volatile fatty acid patterns in the rumen were modified by quebracho tannin, possibly due to it inhibiting microbial fermentation.
The effects of dietary quebracho tannin on the tissue metabolism were investigated further by feeding the material to rats. Voluntary feed intake was reduced in the tannin-fed animals and the apparent digestibility of dietary nitrogen was also reduced. Consequently, N retention and body weight gain were lower in the tannin-fed animals. The weight of three different muscles as a proportion of body weight was not changed by quebracho tannin but there was a marked reduction in body fat. Surprisingly no effect on the fractional synthetic rate of the protein in the mucosa obtained from the first 30 cm of the duodenum was seen.
The conclusion of the study is that there is a prima facia case that some tannins when ingested by ruminants have marked effects both on rumen and digestive function and also on tissue metabolism.
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