If you're going to feed corn at all, feed corn that has been processed
in some way (cracked, rolled, etc) as this increases the digiestibility.
Same for barley, but not so for oats---horses with good teeth can grind
oats just fine, so digestibility is not increased by processing (though
price and rancidity does increase).
Corn does not "heat" a horse in the way you're thinking, ie summer heat.
Soluble carbohydrates have very low heat increment, which is the heat
produced by the body in digesting fiber. High fiber feeeds, like hay,
do have high heat increments (depending on the fiber content) but as the
fiber in grain (and particularly corn) is relatively low, there is
almost no true heat produced by feeding it. Grains are a high energy
feed, however, so some people confuse "energy" with "heat".
As far as how much to feed, every horse is an individual and there are
alot of factors which will influence the amount. How hard he's working,
his size, your size and weight, the terrain you're working him on, how
long he's out for, etc, all added to the fact that every horse is
different. I have a TB mare that gets 8 pounds of grain a day, plus
oil, that by all rights should be as big as a house, but barely
maintains weight just standing around. Another horse works his fanny
off and gains weight on half a pound of grain.
If you're just starting, start out by feeding a small amount, let him
get used to it (don't make big changes all at once) and use your own
judgement in whether he's losing weight, gaining or looks good.
If your horse isn't digesting whole oats well (ie, you see alot of
undigested grains in the poop), you might want to have his teeth
checked, and consider adding a probiotic to help digestive efficiency.
Breaking feed into more, smaller daily meals also increases feed
efficiency, ie feeding grain at noon time instead of on top of two other
"main meals"---more natural to the horse's digestive physiology. Also,
if you're feeding ALOT of oats, remember digetibility tends to decline
as amount increases, it's sort of a point of diminishing returns on
grain digestibility. This also applies to most other feeds as well.
It's normal to see some undigested grain in the poop, if there's alot
undigested, there may be another reason.
Sorry if I'm a little disjointed in my sentences. I got about two hours
sleep last night bundling the Significant Other off to South Korea
(poor blighted soul), classes and research all day, taught a lecture
this afternoon that I may or may not have been awake for <g> and now I'm
falling asleep at the keyboard. So off to feed horsies and then
Hope this helps a bit. Good luck!