Although horses are essentially fibre fermenters, they can of course digest starch, just not in huge quantities at one time.  Recent research has found that no more than 1g per 1kg of bodyweight in starch should be fed at any one meal for sensitive horses, albeit other research has suggested 2g per kg of bodyweight.  If you are unable to adopt this practice and are concerned about providing your horse with enough energy for his work – try replacing some of that grain/hard feed with Fibregenix Platinum Pro ration balancer supplement and adding in a calorie dense feed such as oil or feeding a low starch grain such as lupins, or adding in some super fibres such as beet pulp.  Too much starch in one meal can cause a multitude of problems. From gas colic to ulcers to hot and fizzy behavior, toxicity in the hindgut leading to laminitis, etc etc