You won't want to drop her feed intake after the foal is born-- she will be needing the extra feed to produce milk. In fact you will probably have to keep increasing her feed as the baby gets bigger and nurses more. Its really tough to get weight back on a nursing mare if they start to lose it, and nursing can take alot out of a mare if they aren't getting all they need.
If your pasture grass is fescue, that could cause a problem as far as grazing during the later stages of pregnancy. Your county extension agent might be able to help you find out about fescue in your area, and/or where to take a sample of your grass to have it looked at.
Based on what you have said about her foals, her being a roaned blanket rather than a fewspot does seem likely-- that does not mean all or even most of her foals will roan though-- for example, my stallion above ^^ roaned, but most of his offspring have not. He sires mostly blankets that do not roan, with an occassional roan-- for example, these four adult geldings/stallions are all sired by him, out of Thoroughbred mares--
And the mare on the left has an Appaloosa dam-- the mare on the right has an AQHA dam.
So, depending on the mare, stallion, and serindipitous Appaloosa luck, your offspring colors/patterns may vary