And bear in mind that milk has a lot of fat in it...which can only come from one of two places. It will either come from your mare's diet, or it will be pulled from her own reserves.
My suggestion would be to mix about 1/4-1/2 cup of pure corn oil into her feed, 2X a day. Reason being, corn oil is highly digestable and it contains 120 calories per spoon-full.
Walmart has their GV-brand corn oil at $2.98 per 48 OZ bottle.
And don't neglect her required roughage. I'm in your area of Texas and right now, folks are baling-up their second-cut hay. There's been ads for good hay posted in the Houston Chronicle, for the past few weeks.
Hope this helps.