look for a low grain or no grain formula ...
if you just want to give them something look for a ration balancer they are high nutrition, low nsc(sugars/starches), low calorie and low feed level so cost about 50 cents a day (think Flintstone vitamins or one a days) and no need to worry about them getting HOT at all on one..
If you need help finding one in your area give me your zip code and I can help
Being that you are in Iowa I would bet you can get Kent's Horsego32 which is the one I use
I have been called the NSC Nazi more then once ... I hate traditional feed methods of loading our horses up on grains and junk food :)