Introduce gradually into diet over a two week period. 
Add to feed, split into two feeds (morning and night) where possible. 
Always provide access to clean water.

1 scoop = 10 g

Morning feed: 1/2 - 1 + 1/2 scoops (5g - 15g)
Night feed: 1/2 - 1 + 1/2 scoops (5g - 15g)

Recommended rate based on a 500kg horse.