If its not present in your paddocks check your hay. If you buy form a local Farmer ask him if he uses weed killer to prevent ragwort, if you are lucky to see the field before they are cut so you can see for yourself what condition the fields are in. The best way to buy is from a reproctial farmer that you can trust rather than from a bargain from an unknown source. If you find ragwort in your paddocks take time to go round and dig up the roots and put the weed in a plastic bag, this is to help prevent seeds dropping and either burn the weed or take it to your local tip. The affects of the poison are horrific the symptoms are very distressing and hard for those who are treating it.
Death is inevitable so prevent any more distress the horse is destroyed.
So take some time to go around your paddocks and always check your hay if you think that your horse has eaten any contact your Vet as soon as possible. There are many species of ragwort, a weed that is poisonous to cattle. Farmers try to eradicate this plant by pulling up the young flowers before they can seed, but the seeds can remain dormant in the soil for many years, so it is a continuous job. The plant Senecio jacobea is a hardy biennial. It flourishes in waste ground and roadside verges from where it invades nearby fields.