What about frosted alfalfa, clover, and grasses?
Other common forages such as alfalfa, clovers, and cool-season perennial grasses do NOT produce toxic compounds after a frost. There is different concern, however, when grazing alfalfa or clovers after a frost - and that is bloat. One or two days after a hard frost, the risk of bloat is higher for animals grazing legumes. The bloat risk is highest with grazing pure legume stands, and least when grazing stands having mostly grass and only low amounts of legumes present.
The safest management is to wait a few days after a killing frost before grazing pure legume stands - wait until the forage begins to dry from the frost damage. It is also a good idea to make sure animals have some dry hay before being introduced to lush fall pastures that contain significant amounts of legumes.