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 definition that is used by scientists to identify substances that are "extremely toxic" and it is defined as something where 5 miligrams per a kilogram of body weight or less is lethal. There are one million milligrams in a kilogram so we can say that something that is "extremely toxic" is a substance where 5 millionths of the animal's weight ( or less) being ingested is likely to kill it.
The reality is that the lethal dose is much closer to 5 percent of the body weight. This is 5 parts in a hundred.
This means that the British Horse Society is over estimating the toxicity of ragwort by around the order of ten thousand times!