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I have seen fields with horses in them & ragwort everywhere, made me cringe. If they had seen a horse dying from eating it they would soon get rid of it.
The council never pull it up here either.
Yew is also poisonous as you said, but doesn't as often get into fields.
Ragwort use to be a notifyable plant if i remember correctly. I think I have seen old pre war posters exhorting one to notify the police.
Its should be pulled up and burnt or composted to oblivion. Not having tasted it but observed stock ignore it I think its bitter