Farm animals require a number of nutrients in their diet, among which are proteins. Unfortunately for them, these proteins naturally present in grass (pasture, silage or hay) are poorly absorbed during digestion.
When proteins penetrate into the digestive system of ruminants, initially they pass through the rumen, which can be described as pre-stomach. The rumen brakes down these proteins. Proteins are broken down to the point that they are no longer absorbable when they reach the intestine (where the absorption of nutrients takes place through the intestinal walls, as with all other mammals).
This low absorption leads farmers to supplement the rations with food supplements such as soybean meal, most often imported from abroad.
At the end of 1990s it was found that the presence of condensed tannins in forage decreased the proteins’ solubility (which can be considered as their breakdown). These tannins are metabolites naturally synthesised in the leaves and stems by some species of legumes such as lotus, Sainfoins or Coronilla.
Some species don’t contain tannins in the leaves, but in some other organs. This is a case for white clover flowers or lucerne seeds.
Present in the vacuole of plant tissues, in contact with the protein, these condensed tannins create complexes that resist the action of the rumen, and reach the small intestine undamaged. Having reached this point, the complex is broken down due to the lowering of pH, and the protein may be absorbed by the intestinal walls. As a result of the formation of complexes in the rumen and their separation in the intestine, there is a decrease of nitrogen excreted in the urine and an increase of nitrogen present in the faeces.
The presence of tannins equally avoids the risks of meteorisation. As there are few soluble proteins in the rumen, the protein network formation which traps air and induces swelling of the rumen does not occur. As there are tannins in the white clover flowers, but not in their leaves, white clover causes bloating if only leaves are present and does not cause bloating when there is significant presence of inflorescence.
Formation of protein complexes – tannins also occur in the case of silage production, reducing a degradation of proteins in this type of conservation.
If it is possible to manually add condensed feed tannins (whose action is identical to that of endogenous tannins present naturally inside the plant, as we know it), genetically enhancing other forage species such as lucerne or clover can be considered. The presence of tannins in the walls of the seeds of these species suggests that this means of biosynthesis exists and these channels may be activated by transgenesis. However, all research conducted in this area has failed.
Tannins and Parasite Control
The presence of tannins in feed allows for the control of animal parasites, such as strongylus. This biological control has proved particularly powerful in small ruminants, as well as in France as in the West Indies.
The control mechanism has been studied by H. Hoste. He demonstrated that tannins could bind with proteins which form the wall of strongylus. Formation of these complexes leads to a deformation of the wall and reduces their capacity for growth and multiplication.