I agree, grass, mix, or Timothy hay. Orchard Grass is a nice high nutrient hay that's available in much of the West Coast.
If she's only on pasture, you don't need to worry about colic with hay. If you decide to give her some kind of feed, then you'd just need to start slow, with small meals, building to a full meal over a 2 week period.
The hay should really be enough though. Feed free choice and see how she does. If she still needs more weight, add in beet pulp (1-4 lbs dry weight a day), a vitamin supplement, and some stabilized rice bran or flax (1/2 to 1 cup a day).