There are several species of buttercups and they are among the most common weeds. Consuming any part of the plant except the seeds produces blisters or inflammation around the mouth, irritated skin, stomach pains, vomiting, diarrhea, and jerking spasms. Irritant juices may severely injure the digestive system. Other symptoms may be temporary blindness and convulsions.
Apparently buttercup is non-toxic when it dries so it is not a concern in hay.
Brian and Jane Pinkerton
29343 Galahad Crescent
Canada V4X 2E4