A friend of mine had a horse founder due to too much alfalfa hay. If you can get bermuda hay, that would be good. It's not as rich as alfalfa, so they can eat all they want of it. I'd recommend getting Sandblast as well, add that to their grain one weekend a month and that will help clear out the sand. I live in Florida, so I know all about sand colic. Keep mineral oil handy as well, it's a good colic-preventative. On a very dry, dusty day, you can add a small cup of it to their grain and it helps keep them cleaned out.

I agree with watering down the hay, my horses tend to prefer it that way since it keeps the dust to a minimum.
Image
-----
Holly

0.1 Lavender California Kingsnake (Lizzie Borden) (missing )
1.0 Florida Kingsnake possible mix (Eddie Gein)
1.0 Bearded Dragon (Charley Manson)
1.0 Orange Marmalade Cat (Oliver)
1.0 Egyptian Arabian (Bagan)

2.0 Toddlers (Justice & Trevor)