Dr. Beth Valentine has done extensive research on draft horse diets. Here is one of her articles. Feeding Draft Horses: The EPSM Diet - EC Magazine Fall 2005
Drafts aren't that different than regular horses when it comes to feed. My adult Percheron gets the same as the other horses in the barn.
Read as much as you can about the high fat diet for draft horses. And don't give any grain. Beth Valentine's Recommended EPSM Diets
From an article on The Horse:
A good, balanced diet is especially important for draft horse health.
"There's a lot of bone they're putting down," Stone points out. Draft horse feed proportions are the same as those of the light horse--about 2% of their body weight per day. However, because of the draft's predisposition to myopathies (muscle disorders) such as equine PSSM, Stone says it's important to feed draft horses diets high in fat and fiber and low in carbohydrates (i.e., reduced sugars and starches), with adequate amounts of balanced minerals.
Valentine cautions that when feeding a high-fat and -fiber diet to youngsters, it's important to ensure the horse is not getting so many daily calories that he gets fat or grows too fast.
"A young draft on this type of diet may need a daily supplement to ensure adequate vitamin and mineral intake, as the higher calories of fat will mean a lower volume of concentrated vitamin and mineral fortified feed will be fed," she says.
"Growing horses require more of many of the vitamins and minerals than do adult horses," she says. In general, she recommends feeding plenty of high-quality hay or pasture, a forage-based feed with added fat, and any necessary supplemental vitamins and minerals for draft horses of all ages.
"Vitamin E supplementation--at least 1 IU (International Unit) vitamin E per pound of horse--is also important for any horse not on high levels of alfalfa products or green grass pasture for much of the year," she states. The Horse | Heavy Horse Health Problems