Hemp is an incredibly high quality source of protein, and is gaining traction in the horse world for its protein and omega fatty acid profile.  Its protein contains all the essentail amino acids and is a brilliant alternative to soybean meal. 
 
Hemp protein provides slightly less lysine than soybean meal but more methionine. Hemp also provides the branched-chain amino acid, leucine in higher levels than whey protein. Hemp contains decent quantities of several important vitamins too.  
 
Compared to flaxseed oil, hemp seed oil provides a lower relative amount of omega-3, but it contains that somewhat unique anti-inflammatory GLA that is not found in flaxseed or other oils/seeds commonly fed to horses.
 
Hemp meal is about 50% fibre, 35% protein and around 11% oil. 
 
Of course, how much you need to feed really depends on the rest of your horse's diet, but it's suggested that doses are approximately:
 
100-150gms per day as a maintenance dose
200-300gms per day for horses that need extra protein, amino acids and omega fatty acids
 
Hemp is a brilliant things to feed over summer when the grass is brown and dry and contains very little protein and omega fatty acids.