We very much believe in keeping feeding simple. Our three stocky, Paint mares, 6 yrs (14.3hh, 900+ lbs., short backed and leaner), 8 yrs (14.2hh, 1100 lbs., halter type body built like a tank), and 16 yrs (15 hh, around 1050 lbs., penner) old have always been on only good free choice hay (fescue/orchard around here), a mineral block, and only one cup of Nutrena SafeChoice feed a day or after a ride (which is really just for a treat). They are all lightly worked pleasure trail horses, healthy, low maintenance easy keepers, and have good feet and teeth. Even though they are only lightly worked, you can take them out on the trail all day (W/T/L) and they have plenty of energy to spare. You can look at their pictures...they are not fat, but are certainly 'well fed'
You can certainly cut out the grain and they'll be just fine.