Upping the fiber content in his diet is where I would start before I added expensive supplements. Horses are designed to eat LOTS of fiber and in the winter digesting the fiber will go farther to keep his body heat up than grain or supplements. So I would say up the grass hay..keep him full. Free access to hay is the best thing for you horse.
A close second would be soaked beet pulp. Again, it ups the fiber and fills his belly. Just be sure to rinse any molasses the feed mill may have added. Sugar and starch are NOT good for your horse in large quantities. You can safely feed up to 10 lbs (beofre soaked) of beet pulp a day, though you probably won't need to feed that much. Grains and prepared feeds I generally view as supplements. Because almost all horses if they get enough grass hay have no weight issues except when in hard work then I think some oats and sparing amounts of alflalfa are fine since the horse's eating time is spent by the work time. But going into winter I wouldn' t add more concentrates to his diet, chances are he'll be more idle, not more active. Again, hay is your best option to help with the weight.
A bonus to giving more hay is your horse will be more content throughout the day and less apt to be cranky or develop bad habits out of boredom since hay takes longer to chew, it's more time consuming. Another bonus...less chance for colic and founder, since the tummy stays full more of the time and there's no blood sugar spikes and less worry about founder since the sugar/starch that can cause it isn't a strong factor.
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