Take a look at Katy Watts | Safergrass.org
& Horse Nutrition Facts and Equine Feed Information
for general diet/nutrition info. www.Feedxl.com
is a brilliant resource IMO to help work out what particular horses in particular situations need & gives access to equine nutritionists to ask specific questions of. If it's your own property, I'd personally do a pasture analysis to know precisely what is there/lacking.
Basically, keeping her diet as simple & natural, and adding a good nutritional supplement is generally best. Ie horses are built for near constant feeding on small amounts of poor quality(compared to cattle fattening pasture/grain), fibrous roughage. So average grass/grass hay is generally a great base.
Supping either a powdered complete balancer as Annabel suggests, or a (grain free) pelleted 'ration balancer' can be the simplest way to ensure they're getting good nutrition. Eg. My horses are on pasture without need of additional feeding. I've found a suitable low dose, low-cal 'ration balancer' that gives them everything they need in one small handful, aside from being a bit short in sodium, chloride & iodine. So I supply that with a lump of rocksalt with a bit of liquid seaweed emulsion poured over it, free choice in their paddock. Omega 3s/fatty acids are something that don't come up in analysis but I consider important & I supp these with a small amount of freshly ground linseed.
You will see in the above sources that after more recent research, grain & other high sugar/starch feeds are often considered not the best any more. The reason is that horses don't cope well with too much starch/sugar & this can cause a number of health issues. Unprocessed grain is also generally hard to digest and when it's fed in only a few or less meals a day, rather than little & often as the horse's GI tract is built for, it can be even more problematic. There are IMO generally more appropriate, safer forms of high energy feed, such as beet pulp, alfalfa, etc if necessary.
But on that note, my bet is too many calories will likely be her problem & she won't be likely to need extra. Instead you'll likely need to restrict intake. This can be done as annabel suggested, with part time use of a grazing muzzle, keeping her in a grass free area & feeding soaked(to leach out sugars) hay, or keeping her on a track setup. The latter is what I choose, as it also motivates more exercise for otherwise sedentary paddock dwellers. You can get some good ideas of different possibilities & compromises of this type management on SNHC*|*Paddock Paradise Grazing