Prevention is better than cure:
Pull out Ragwort and remove any acorns from paddocks.
Check paddock boundaries for evidence of poisonous plants, check in hedges and on the other side of the boundary to a distance a horse could reach.
Make sure that your boundaries are secure.
Break open hay bales and discard any plants that you cannot identify.
If you suspect poisoning always call your vet immediately.
Disclaimer: The list of poisonous plants on this web site does not necessarily include every poisonous plant that is known, or that might be found in an urban landscape or home.