You want to ensure that your horse is well conditioned before SLOWLY reducing the amount of hard feed that you feed to your horse. While slowly reducing the amount of hard feed and allowing him/her to graze, you should check their weight and the condition of the horse to see whether he/she is losing condtition as the grass may not be enough for him/her thus leaving him with the same amount of hard feed - when you begin to notice that your horse is becoming abit more "weighty" for example from the grazing, then you should be able to decrease the feed slowly. You can tell when the new spring grass grows when the shoots come out - this can help you decide what to do! Hope this helps!