Thanks, Sue, I have looked at those sites too - plus the Merck Vet manual etc. We do not have ragwort in any of the pastures. I have combed their pasture, sent feed off for analysis (negative), examined and cleaned the water trough (which had healthy fish swimming in it). This mystery plant is the only weed that the horses definitely ate that I could not identify and clear. However since it isn't on anybody's toxic list, I think it too is and unlikely culprit. I'd still kind of like to know who it is; it is my favorite weed...