There is no requirement for omega-3 fatty acids in  horses. Humans should consume about 1.6 g of omega-3 in the diet per day. Using that amount as a basis for a recommendation for a 1000-pound horse, the intake should be around 8 grams of omega-3 each day. The amount of flax you are giving one-half cup will supply the horse with approximately 12.8 grams of omega 3 per day. If the horse is happily eating this amount, then I would not change it.