How dangerous are plants like curly dock and white and yellow sweet clover for sheep? Some sites have them as lethal and others don't mention them in the lists of dangerous plants.

Helen, According to "A Guide to Plant Poisoning of Animals in N. America by Anthony Knight and Richard Walter, oxalates are the primary toxins in curly dock (Rumex spp). Although not a common source of oxalate poisnoning, it will cause poinsoning if eaten in excess by livestock. As for sweet clover (Melilotus), it in itself is not poisonous, however, if it becomes moldy, a variety of different fungi are capable of converting coumaris to dicoumarol. However, the book specifically states that sheep appear quite resistant to the toxic effects of dicoumarol.