Campbell, A. and Chapman, M. (2000) Handbook of Poisoning in Dogs and Cats, Blackwell Science, is a comprehensive dedicated veterinary reference work covering a wide range of poisons relevant to dogs and cats. It is comprehensive enough to include entries for Silica gel, Poinsettia, and Honeysuckle. Yet ragwort does not appear in it, nor does Senecio, nor pyrrolizidine alkaloids in any form. The absence of ragwort from a handbook this comprehensive, covering risks this minor, is a strong indication that it is not considered a recognised veterinary risk to dogs in the scientific and clinical literature.
Critically, a paper published in Nature established that mould-related liver pathology is "indistinguishable from ragwort poisoning" (Sargeant et al., 1961). Dogs are far more plausibly exposed to hepatotoxic damage from mouldy food, including poorly stored dry food , than from consuming ragwort in the field. " For more details on the absence of a definitive test click here
The reality is that a search of the scientific literature shows not one single case of ragwort poisoning reported in dogs or cats. There are plenty of scare stories in the press from time to time but there is no evidence that ragwort poses any serious risk at all to dogs.
Ragwort is of course poisonous to mammals. So if a dog (or a cat) were to be force fed this intensely distasteful plant it would be poisoned. However, the poisonous dose for the grazing animals where it is a risk if it is in hay is measured in percentages of body weight so it would be safe to assume that the same sort of dosage risk would apply to dogs (Goeger et all 1982). There would be no risk to them from just encountering the plant.
One of the issues is that there are vets who are so concerned to warn people about ragwort that this overrules telling the truth, often due to a surprising lack of critical thinking skills in examining the facts.
The message here is just don't panic about this scare story. Look at the facts of which you will find plenty on this website, and make your decisions on the basis of the evidence.