The British Horse Society falsely claimed in a 2014 survey that "ragwort is extremely toxic to horses." The survey contained many serious faults and is very poorly constructed but here we are looking at the scientific validity of that statement.
There is a precise scientific definition of "extremely toxic." According to the Environmental Health and Safety department of the Georgia Institute of Technology, a substance is classified as "extremely toxic" if 5 milligrams or less per kilogram of body weight is lethal (Georgia Institute of Technology EHS, 2024). There are one million milligrams in a kilogram, so an "extremely toxic" substance is one where 5 millionths of the animal's body weight or less is sufficient to kill it.
The reality for ragwort is very different. Research published in the American Journal of Veterinary Research established that ragwort intakes of between 5% and 20% of body weight are lethal to cattle and horses (Goeger et al., 1982). Taking the lower figure of 5%, this is 50,000 milligrams per kilogram of body weight, which is ten thousand times more than the threshold for "extremely toxic." The British Horse Society is therefore overstating the toxicity of ragwort by around ten thousand times.