Great thread, and lots of great info here.
One thing to keep in mind is that hay fufills several needs; only one of them is strictly nutritional. Hay also provides bulk or roughage that keeps the horse's gut working correctly and for lack of a better term, entertainment. Hay occupies the horse while stalled or when grazing is not available and keeps them chewing and contented. (Chewing releases endorphins.)
So while the Equine Senior may be a complete food nutritionally, it might be wise to add hay cubes, beet pulp or another form of roughage to her ration. It's really a question for your vet; you don't want to add something that will upset the nutritional balance of the feed. Once you have good spring grass available, it won't be as much of an issue.