High concentrations of tannins in fodder plants inhibit gastrointestinal bacteria and reduce ruminant performance. Increasing the proportion of tannin-resistant bacteria in the rumen protects ruminants from antinutritional effects. The reason for the protective effect is unclear, but could be elucidated if the mechanism(s) by which tannins inhibit bacteria and the mechanisms of tannin resistance were understood. A review of the literature indicates that the ability of tannins to complex with polymers and minerals is the basis of the inhibitory effect on gastrointestinal bacteria. Mechanisms by which bacteria can overcome inhibition include tannin modification/degradation, dissociation of tannin–substrate complexes, tannin inactivation by high-affinity binders, and membrane modification/repair and metal ion sequestration. Understanding the mechanism of action of tannins and the mechanism(s) bacteria use to overcome the inhibitory effects will allow better management of the rumen ecosystem to reduce the antinutritional effects of tannin-rich fodder plants and thereby improve ruminant production.

Bacterial mechanisms to overcome inhibitory effects of dietary tannins.

Tannins (hydrolyzable and condensed) are water-soluble polyphenolic compounds that exert antinutritional effects on ruminants by forming complexes with dietary proteins. They limit nitrogen supply to animals, besides inhibiting the growth and activity of ruminal microflora. However, some gastrointestinal microbes are able to break tannin–protein complexes while preferentially degrading hydrolyzable tannins (HTs). Streptococcus gallolyticus, Lonepinella koalarum and Selenomonas ruminantium are the dominant bacterial species that have the ability to degrade HTs. These tanninolytic microorganisms possess tannin-degrading ability and have developed certain mechanisms to tolerate tannins in feeds. Hence, selection of efficient tanninolytic microbes and transinoculation among animals for long-term benefits become areas of intensive interest. Here, we review the effects of tannins on ruminants, the existence and significance of tannin-degrading microorganisms in diverse groups of animals and the mechanisms that tannin-degrading microorganisms have developed to counter the toxic effects of tannin.

Interaction of gut microflora with tannins in feeds

http://directory.umm.ac.id/Data%20Elmu/jurnal/A/Animal%20Feed%20Science%20and%20Technology/Vol86.Issue1-2.Jul2000/10518.pdf

Chemical composition and degradation characteristics of foliage of some African multipurpose trees

In the tropics, animal production is compromised by the lack of suitable forage for the animals. As a further constraint, low-fertility, acid soils are prevalent in areas where many small and medium size farmers are dependent upon livestock agriculture as their main source of income. This prevents the incorporation of high quality forage legumes (i.e Leucaena leucocephala, Gliricidia sepium) into those grazing systems, as these legumes do not perform very well under those conditions. In turn, most of the tropical forage legumes with good adaptation to acidic soils are of low quality and poor acceptability by livestock, which might be associated with their high contents of condensed tannins. However, research on the impact of condensed tannins upon the nutritional value of forage has not provided unequivocal results. This could be related to the variation in structure (molecular weight and monomeric composition) observed among condensed tannins from different legumes. This study investigated the impact of condensed tannins from different tropical legumes on the degradation of different substrates by rumen microorganisms and/or their enzymes under the scope of structure-activity relationships. In an initial experiment, the chemical composition and in vitro degradability of immature and mature forage of seven tropical legumes (Calliandra calothyrsus, Clitoria fairchildiana, Desmodium ovalifolium, Flemingia macrophylla, Leucaena leucocephala, Leucaena macrophylla and Leucaena pallida) were determined. Chemical composition varied greatly among the different samples. In turn, gas pool after 144 h of fermentation ranged from 66 to 140 ml and in vitro degradability ranged from 230 to 630 g kg of dry matter. The concentration of condensed tannins was associated with a decrease in the initial rate, but not with the extent, of dry matter degradation. In particular, the concentration of condensed tannins had significant negative relationship with the maximum rate of gas production observed during the fermentation of these samples (p ≤ 0.0011). Results from this experiment suggested that condensed tannins differ in nutritional effects and that factors other than condensed tannins affected the in vitro degradation of the legumes studied. In a second set of experiments, condensed tannins purified from all tanniniferous legumes differed in molecular weight, (number-average molecular weight = 2360 to 4880 dalton). Condensed tannins also differed in monomeric composition, with some

Condensed tannins in tropical forage legumes : Their characterisation and study of their nutritional impact from the standpoint of structure-activity relationships. Thesis (Ph.D.)

http://directory.umm.ac.id/Data%20Elmu/jurnal/A/Animal%20Feed%20Science%20and%20Technology/Vol81.Issue1-2.Sep1999/10406.pdf

Effects of ruminal inoculations with tannin-tolerant bacteria on fibre and nitrogen digestibility of lambs fed a high condensed tannin diet

This study examined the distribution of Streptococcus caprinus in domestic and feral ruminants, and the effect of rumen inoculation with S. caprinus on nitrogen digestion in mulga-fed sheep. S. caprinus is a tannin-resistant rumen bacterium found in feral goats and may play a role in alleviating the nutritive depressing effects of high concentrations of plant tannins in these animals. S. caprinus was present in samples of rumen fluid from feral goats and feral camels but was not detectable in rumen fluid from sheep and domestic goats that had not previously grazed forages containing condensed tannins. A proportion of sheep, domestic goats, and cattle grazing in mulga paddocks, with either feral goats or camels, had detectable populations of S. caprinus. Pure cultures of S. caprinus grown in vitro were successfully placed in the rumen of sheep; however, nitrogen digestion was unaffected by rumen inoculation. Interactions between several populations of micro-organisms may therefore be involved in achieving improvements in protein digestion.

Streptococcus caprinus is ineffective as a rumen inoculum to improve digestion of mulga (Acacia aneura) by sheep

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Streptococcus caprinus is ineffective as a rumen inoculum to improve digestion of mulga (Acacia aneura) by sheep