DEER AND TANNINS

This article was originally printed in the August / September 2001 issue of Tracking The Industry. . .
This article is copyrighted and may not be reproduced in any form without permission.

Tannins are a family of polyphenolic chemicals found in bark, leaves, fruits, nuts and seeds of many broad-leafed plants. There are two broad categories termed hydrolysable and condensed tannins which differ in their chemistry and nutritional effects.

For domestic animals, tannins usually are considered to be toxins or, at least, antinutritive substances. Dietary levels exceeding 6% on a dry matter basis will kill pigs and poultry and impair the performance of cattle and sheep. On the other hand, small amounts of tannin (several percent) sometimes are beneficial, improving performance of ruminant livestock. For humans, tannins seem to account for the healthful effects of tea and red wine (taken in moderation!). A comprehensive internet site on tannins and other plant chemicals is maintained by Cornell University (http://www.ansci.cornell.edu/ plants/toxicagents/tannin/tannin.htm).

In evolutionary time, deer and other browsing ruminants have adapted to tannin-rich diets and even prefer the astringent bitter taste associated with high tannin content. Odocoileine deer (mule deer, white-tailed deer) have proline-rich saliva which binds tannin allowing them to thrive on diets that contain over 20% condensed tannin.

Recent practical experience and scientific research suggests that not only do deer tolerate tannin, they may do better on tannin-rich diets. In Korea, deer often are fed oak leaves or forest by-products, obstensibly to simulate natural diets. In Britain, deer farmers report that deer seem to be healthier on pastures comprised of sainfoin and other tanniferous legumes.

Scientific support for these husbandry practices comes from studies at Massey University, New Zealand, where the benefits have been clearly shown for red deer grazing chicory, lotus or sulla pastures. Their studies suggest a number of reasons for these positive effects. One of the most obvious is improved nitrogen utilization by protecting feed protein from ruminal degradation. Tannins bind forage protein at neutral pH (the rumen) and release it in the acidic environment of the abomasum. This so-called by-pass protein can increase the efficiency of nitrogen utilization. Tannins also modulate the microflora of the digestive tract and this may improve feed utilization. An interesting additional benefit is reduced dependence on anthelminthics to control gastrointestinal parasites.

Preliminary studies using purified bark tannins at the University of Alberta's Ministik Wildlife Research Station suggest that these natural chemicals are particularly important for odocoileine deer, especially mule deer. A further outcome of the study was that mule deer did not adjust well to diet change and probably would do best on pasture supplements that do not keep changing throughout the year. A completion report submitted to project funders, the Alberta Agricultural Research Institute and the Alberta Deer Association, is available on the internet (http://www.deer.rr. ualberta.ca/library/tannin/ ).

Elk and red deer are mixed feeders and have adapted quite well to seeded pastures. Farmed white-tailed and mule deer are obliged to adapt to pastures that do not mimic natural habitats. Odocoileine deer do not digest grasses efficiently and broad-leafed plants and grains are digestible, but differ greatly in chemical composition from natural diets. Alfalfa is high in protein and rapidly digested. However, alfalfa is low in tannins and high in saponins that seem to impair rumen motility. This is not to say that alfalfa is not a good pasture legume. My deer prefer it and quickly converted a mixed pasture to a pure stand of grass. However, we probably can do better. Research on tanniferous legumes such as sainfoin for light soils and bird's foot trefoil for northern areas is urgently needed.

Pasture supplements also require attention. If natural browse is available, a pelleted alfalfa-barley supplement is a suitable choice. However, if browse is limited, some attention probably should be given to tannins. Peas, lentils and sorghum are rich in protein and tannins and may be appropriate feed ingredients. However, we also need to take a closer look at alternatives such as commercial extracts from yucca, quebracho and forest by-products such as bark.

A great deal remains to be learned but the rewards may be significant and even critical for sensitive species such as mule deer.

Robert Hudson
Professor of Wildlife Productivity and Management
University of Alberta
robert.hudson@ualberta.ca

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