Keeping weight on an older mare?

Rosie is 20, and although I know that's not freakishly old, she still counts as a "senior".:P
She gets wormed on a regular basis, and she is due for a teeth floating (get paid in a next week!) which I am sure has to do with it.
She has been rather hard to keep weight on this year.
I have bumped up her feed, she now gets two scoops of senior, half a scoop of Omlene 300, and a scoop of soaked alfalfa pellets. (that's the grand total for both morning and evening feeding). We haven't been able to feed hay much due to the severe drought, but we have stocked up for the winter and are prepared with alfalfa, prairie, timothy, actually just about every kind of hay. Whatever we could get our hands on!!!
She gets worked every Tuesday for a couple hours at drill practice, along with trail rides on weekends every now and again, or just fun riding around the pasture.
What do you do to keep weight on your older horses?

Who thinks there should be a thing on her just for senior horses? *raises hand*

Crap. Just saw the "Nutrition" part. Gahhhhhh....

I would recommend putting her on Purina Senior Equine Pellet Feed. It helped wonders with the 25 year old gelding. When we got him he was nothing but bones (he was not neglected) and once we put him on that feed in a few weeks he looked like a completely different horse and acted like one too.

That's actually what we feed her!:)

Add a fat supplement if that's still not working. And free choice hay/pasture.