Buttercup - Ranunculus spp.
      Toxins: All my sources agree that its principle toxin is ranunculin which releases protoanemonin. This can be an irritant when fresh, potentially causing mouth irritation, excessive salivation and diarrhea. Because portoanemonin is unstable and turns into a yellow oil or volatilizes, Cheeke says that it is non-toxic when dried in hay (Natural Toxicants in Feeds, Forages and Poisonous Plants, Peter R. Cheeke, 2nd Edition pg. 449). Both Cheeke and Knight et al ( A Guide to Plant Poisoning of Animals in North America) say that feeding trials indicate that buttercup does not seriously affect cattle or sheep in most circumstances. However, bur buttercup is responsible for many sheep deaths, so do not teach your sheep to eat this.
      Protoanemonin has a bitter taste which can be passed into the milk of lactating animals, so I would caution dairies against teaching their animals to eat this plant unless they do milk taste tests to determine results.
      If I were to train animals to include this food in their diet I would go forward very cautiously. I would watch them closely for signs of irritation and ensure that they have plenty of other forage to mix with it both during training and in pasture. If I saw any negative signs I would stop at once and provide them with intestinal protectants (see your vet) until the plant has been eliminated from the digestive tract. I would bale it in hay as a way of controlling it.