Mine get nothing but pasture, no grain, no additives.
They have a mineral block, that is all.
The broodmares keep excellent weight on grass alone, even when they have foals nursing.
I do feed the foals, separate to the mares, from around a week to ten days old as I like to have them up to speed at weaning.
I think it is this, feeding the foals, that prevents the mares from being "dragged down", since I have always fed my foals, and, by the time they are ready to be be weaned they are eating a LOT, I have never had a mare dragged down by a foal, or lose weight at all.
Feeding the mare just makes her produce more milk, which is not required.
Feeding the foal puts the weight where it is needed.