Why is buttercup bad for horses?

The buttercup (Ranunculaceae family) is potentially poisonous because it contains a compound called protoanemonin, which acts a powerful irritant to almost all grazing animals including horses. However most animals know to keep away from it due to the poor taste of the plant. The most common effect of Protoanemonin is to cause ulseration of the mouth, soreness of the mouth and sometimes colic. This 'effect' is the harmful side effect of the buttercup plant, as the effect stops the horse eating. As mentioned it is unlikely that a grazing animal will eat enough of the butercup to cause this effect due to the plants poor taste. A possible exception to this could be in the case were a horse had limited grazing and was also not getting fed regularly from its owner. When no other food is available to a grazing animal it will tend to eat what is there in order to survive even if it tastes bad. If you are concerned it is always advisable to contact your vet.