My mares get 1st cutting hay, 3 different free-choice salts (like yours), and a fabulous loose mineral called, "Grostrong" - it comes in a 25 lb bag. I would really look into getting some, and within 2 wks you may see quite a difference! Meantime, definitely give Sand-Aid or Sand clear pellets for a whole weekevery month to prohibit sand colic. I'd recommend starting that immediately. Best of luck! :)
I'd be inclined to think she's has some kind of mineral difficiency. I use Hoffman's mineral as free choice. The first month, my mare ate it like candy and then settled down to a more usual 250 gr or so a day.
I agree with getting her checked for selenium deficiency, because that can have very devastating consequences as I just recently met a mare who had had it. She was extremely unhealthy and unsound, and eventually had to be euthanized because of it. Not trying to alarm you, but get her checked. I also have another theory, though. My dachshunds do the same thing, and I had them chcked and tested and everything, to no avail. Then someone (a biologist) told me that there are myceliums (sp?) in the soil- a type of fungus- that they have developed a taste for, kind of like truffles. No idea if this could apply to horses, but just throwing it out there!
Blood work should be done to test for Selenium deficiency. A mineral lick will help. I use a big screwdriver and hammer and break off the corners and break those up into smaller pieces. That's what the horses go for first. Dirt licking can also be a salt deficiency. If you provide a lick, break off pieces or offer loose salt, regular iodized household salt or pickling salt. Pickling is iodine free so a mix is better.
Yeah, like I said, after I broke chunks off the mineral block, ground them up, and measured them into her grain, she stopped liking at the dirt. She was nibbling at that area a couple days ago, but a clump of clover was trying to grow there- she was going for that rather than the dirt.
I had her blood work in with the vet for them to check for a thyroid problem (nothing, thank goodness) and I'll have them check selenium.