Thistles are fine for horses to eat. Make sure they're not the big, spiky typical 'Scottish' type of thistle as I have heard of a horse dying after eating these - colic after a perforated gut. Small prickles are fine, but big spiky, thorny ones might not be. Simply hack the heads off with a pair of shears and they'll be fine.

 All the horses at my barn eat thistles too.

yes it is totally normal for horses to eat thistles; they are very good for the liver and nettles are rich in iron and  vitamins, they are also a good anti-inflammatory and extremely good for laminitics.

When nettles are young, if you cut them & leave them to wilt, the sting goes out of them and the horses will eat them; as they get older and drier, they don't sting so much and the horses will eat them from the stalk.

We are so used to seeing grazing land that is so sanitized that nothing else but grass is allowed to grow, but that is a very unbalanced diet; horses by nature are browsers and grazers, in the wild they would eat a variety of herbage, not just grass.

for the full article go to http://www.bforrestsaddlery.co.uk/grazing,_thistles_&_nettles.htm