Producers in the southern and central United States use it for livestock feed. It's available and fairly cheap, but not as nutritious as tannin-free grains. When Adeola investigated the problem, he found that tannins attach themselves to certain digestive enzymes bound to the membrane of the small intestine. Unfettered, the enzymes snag passing proteins and carbohydrates, break them down into amino acids and sugars, and make them available to go into the animal's bloodstream. When bound to tannins, however, the enzymes are unable to catch and break down passing food molecules.
Potential nutrition passes through instead of being absorbed.
Adeola would like to foil the tannins. But when we understand the whole picture, we may be able to look for agents that could reverse or decouple the binding between tannins and enzymes.