Most common forage legumes are not high in tannins, the major exception being some varieties of serecia lespedeza. As for the warnings on feeding raw legumes, they apply mainly to the seeds - soy beans, beans, peas, etc which should be heated before being fed. Legume hay and pasture has been recommended for hogs and poultry for a long, long time. Also, the fiber content of leaf matter and growing vegatative portions is much lower than stems or non-growing portions.