A few years ago the Highways Agency spent millions of pounds on removal of ragwort from roadsides and they were criticised by the UK taxpayers for wasting money. They were following legislation under the Ragwort Act 2003 which compelled landowners to deal with ragwort. In the past few years, there’s been minimal effort to control or remove ragwort so it seems to have fluorished to the point where you are lucky to travel a few hundred yards in the countryside at this time of year without seeing the typical clusters of yellow flowered plants.
I drive around our practice area quite a lot at this time of year and rarely a day goes by when I don’t see horses grazing in a field contaminated with ragwort. Although most horses won’t eat ragwort plants on purpose, they can consume small amounts accidentally while grazing and the plant toxins can result in long term damage to liver cells which may not become apparent until irreversible damage has already occurred.
Ragwort plants are prolific and will spread year on year if not dealt with. If they contaminate meadows then can end up in hay or haylage which represents a greater risk to horses. My advice is not to take the risk - Deal with any ragwort now.
Detailed information on pasture control can be found on the BHS website under Welfare. http://www.bhs.org.uk/welfare-and-care/ragwort-toolkit-country-selection