You can't miss seeing ragwort as it is always all over road verges, on wasteland, by railway lines and then of course, on pasture, particularly poor pasture. You will see it in the summer - that tall green stem with a yellow flower on the top.

It thrives on poor pastures where there are gaps without quality grass growth, often caused by poaching, overgrazing etc.  However, a good well managed grass sward can prevent the growth of the ragwort due to the tigher growing grasses.

If horses have good quality forage to eat, they will not be tempted by the unpalatable ragwort.  However, where there are too many horses grazing poor quality pasture, the quantity of ragwort increases and, if there is litte alternative, the horses will eat the ragwort.  Apparently, they can then even acquire a taste for it.

Then, of course, comes hay and haylage making time and once the ragwort is cut and dried, the ragwort becomes much more palatable to horses, but, it is still poisonous.  It causes liver failure.

Read more here :

http://www.ilph.org/advice/ragwort.asp

http://www.homeofrestforhorses.co.uk/pdfs/no7ragwort.pdf

http://bhs.org.uk/DocFrame/DocView.asp?id=1089&sec=-1

http://www.defra.gov.uk/rural/horses/topics/ragwort.htm