Tannins, Nutrition and Internal Parasites

Tannins are naturally occurring polyphenols made by the plant to defend it against microbial invasion. There is a huge array of different tannins, all of which have different effects on the animals (and humans) eating them. Many, but not all of them are toxic, and many reduce the digestibility of a plant, particularly its protein. A common characteristic is that they bind to and precipitate proteins, but some also have anthelmintic activity. The beneficial effects of tannins need to be explored, while the detrimental effects of tannins can in some cases be overcome.

Reducing Protein Degradability in the Rumen Effect of Tannins on Digestibility Anthelmintic Activity References and Further Reading

Reducing Protein Degradability in the Rumen

The binding of tannins to proteins can be useful when feeding sheep and goats. If the diet contains too much degradable protein and not enough fermentable energy the rumen micro-organisms will not be able to convert all the degraded protein into microbial protein. The addition of tannins in this situation would reduce the protein degradability in the rumen, and increase the supply of undegraded protein to the animal (see section on digestion in ruminant animals). Provided the undegraded protein is digestible by the animal, then the supply of potentially good quality protein to the animal is increased by the addition of tannin. Such an intervention needs to be carefully planned, however, as the result could be that the reduction in protein degradability only serves to reduce the supply of microbial protein and the undegraded protein remains undigested as well, eventually passing out of the animal in the faeces. Clearly, in this case the supply of protein to the animal would have been reduced rather than increased. Different tannins have different binding activities as well, and so there is no direct relationship between tannin content and protein degradability. This does make the strategic use of tannins to reduce rumen protein degradability a lot more difficult.

Effect of Tannins on digestibility

The most widely known effect of tannins is that of reduced digestibility. They also have an astringent taste, and so feeds with high concentrations of tannins are often unpalatable, as well as being relatively indigestible. The low digestibility of feeds with a high tannin content is a consequence of the ability of tannins to bind with and precipitate proteins. Feeding large amounts of tannins will therefore reduce the amount of nutrients actually being fed to an animal, as the more tannin that is in the diet, the greater the proportion of nutrients will pass straight through the animal and be excreted in the dung.

To overcome this, and some of the other harmful effects of tannins, there has been a great deal of work focused on treating plants having high tannin concentrations in order to neutralize the tannins. Compounds that do reduce the biological activity of tannins include polyethylene glycol, but this is not going to be of any practical significance to resource poor smallstock keepers. However, other work has shown that wood ash can also alleviate the problem of certain tannins, and this is a technology that could be used by smallstock keepers feeding forages and other feeds with a high tannin content.

A number of plants contain tannins that have anthelmintic activity - the ability to reduce the problem of intestinal worms. This means that the inclusion of these plants in the diet of small livestock could be done strategically (at key times of the year) to control infection by internal parasites. Many of the tannins that do have anthelmintic activity are also harmful to livestock as well, which is why their inclusion in the diet should be controlled and only done for short periods of time.

The effectiveness of this measure depends to a large extent on identifying the tannins that are most suitable for use as alternative anthelmintics, identifying the plants that contain these tannins -and which are available in the area in question, and then identifying how much (and of what parts of the plant) should be given to different classes of livestock to bring about required reductions in the parasite population.

Considerable work still needs to be done to develop this technology, but some useful guidelines have been developed. See the links below for some of the relevant research projects.