I found this:
Has long been used in the UK and Europe to enhance coat condition not only as a wash but as a feed additive.
We prefer to use it internally as it is also a good general tonic.
Nettle can still sting when it is dry so be sure to wet it before feeding to your horse.
Nettle has traditionally been thought of as an anti-inflammatory but only recently has its mode of action been demonstrated and scientifically confirmed.
Its inclusion in your horses feed will therefore help to prevent inflammation and subsequent damage to the joints.
Nettle is also an excellent diuretic and through this action aids in detoxification.
Although some commentators believe that nettle has some antiviral or antibacterial activity, we have found no scientific evidence to support this activity. This apparent effect may be due to its nutritional value.
How much and how often would you/do you feed nettle?