Horses although being essentially fibre fermenters, do have a digestive system that can 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 although other research has suggested 2g per kg of bodyweight – If you are unable to do this – try replacing some of that grain/hard feed with Fibregenix Platinum Pro as above and adding in extra oil.  Too much starch in one meal can cause a multitude of problems, anything from gas colic to ulcers to hot and fizzy behavior, toxicity in the hindgut leading to laminitis, etc