I just built a 24x32 pole barn and need to backfill the inside. What would be the correct fill ? We are using it for horses.

Some questions
1) Where is the barn located? Is it heated or might frost be a problem?
2)Are you putting a concrete pad down as the main floor material?
3) is water drainage likely a problem?
4) what is the soil underneath? Do you have a good base to start with?
5) what height of backfill do you need to add? Do you have any material handy that you would like to use... or are you having to bring everything in.

Frost is not a problem, there will be a concrete isleway,no water troubles, the soil beneath is sand, about twelve inches or so, probably will have to truck material in

Dirt or sand floors makes washing down the horse stalls hard, wood is the best flooring for them, concrete is not a good choice. Just make sure to provide a straw bed for them whatever you use.

Sand can cause horses to be affected with colic. I would use clay fill packed down, the rammed earth system but only because it is plentiful in our area and dirt cheap. Or maybe pea gravel. You don't want anything that could be slick when wet that could lead to leg or foot injuries, especially in a breeding stable. If a stallion slips and falls he is less likely to do well for you if his feet hurt him.
You could lay stall mats overtop of whatever but they are pretty expensive

These answers are appreciated, the stalls will be compacted crushed limestone with mats, just looking for good ideas for base fill for elevation

Good one

That could be sand or whatever fill you can get cheap then, crushed limestone and mats will be great.

I live in a pole house I built 20 years ago. The poles are untreated. http://www.photobucket.com/wwpierre

Make sure you don't use anything which will tend to hold moisture against the poles. As long as they are kept dry for the first foot or so below ground level, they will last.