Tannins are naturally occurring plant polyphenols. Their main characteristic is that they bind and precipitate proteins. They can have a large influence on the nutritive value of many foods eaten by humans and feedstuff eaten by animals. Tannins are common in fruits (grapes, persimmon, blueberry, etc.), in tea, in chocolate, in fruit dish.legume forages (trefoil, etc.), in legume trees (Acacia spp., Sesbania spp., etc.), in grasses (sorghum, corn, etc.).
Tannins contribute to many aspects of our daily lives. They are responsible for the astringent taste we experience when we partake of wine or unripe fruits, and for the enchanting colors seen in flowers and in autumn leaves.
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Nutritional Effects: positive effects
The presence of tannins in food sources for monogastric animals, is generally viewed adversely, though their contribution to red wines is certainly an exception. However, in ruminants, tannins can induce beneficial effects. For example,
In sheep and cattle higher retention of nitrogen has been observed in sheep and cattle with low to moderate levels of tannins in forages,
In these cases, the lower apparent and true digestibility of nitrogen was compensated for by reduced urinary loss of hydrogen,
Moderate levels of tannins (less than 4% ) in forage legumes can have beneficial responses in ruminants, resulting in higher growth rates and milk yield,
However, even in ruminants, levels of tannins exceeding 6% of the diet result in negatively affect growth rates and milk yield.
Several mechanisms have been suggested to explain how tannins influence protein utilization by ruminants -
