Sorry this is late it's been crazy with school starting.  I use the scoop that came with a bucket of sand clear for the daily dose, it holds about 2 tablespoons.  When I find sand I feed a pound a day for 5 days and then half a pound until the poop is clear.

Thanks for everyone's tips... So far so good for us... I've been keeping the horses stalled during the day.. Bought about 50 bales of hay and keep it in my garage.. I haul a bay over to the barn every 2-3 days.. My girls have gained weight, all of the scrapes from fighting at the community bale are mostly gone and Their coats are dark and shiny from staying out of the sun.. Keep your fingers crossed for me..

Leslie

Here's what I do: 1)I feed SandClear (or another product that's cheaper) from the 1st through the 7th of each month. 2)I feed probiotics the first 3 days of the 'SandClear week' and every Sunday during the month. 3)I feed 1TBSP of plain table salt each night (this is in addition to free choice blocks) to ensure/increase water intake.  4)I feed grass hay with occasional forage as a treat. My horses graze in a sandy pasture during the afternoon, so I feed them in the barn on mats morning and night. So far, so good.

Last month we went on vacation and I had the neighbor feed for me. I had an old bale with some mold on it that I'd put outside the barn on the other side of the corrals ready to spread when the rains arrive. It was about 20 yards away from the barn where I store my hay inside, plus it was oat and I feed grass. Well shoot, I'll be darned if the neighbor didn't decide to "clean up" the old moldy bale and feed it to my horses!!! I nearly choked when I discovered what had happened. So far the horses are fine. So, even though I'm super-vigilant about my feed, I lost control and lucked out. WHEW! You just never know.

 Some really good advice so far; here's my .02 worth....

  1)MATS under where they eat! My stalls are matted,plus, I keep one or two mats under where they eat outside in their runs; I sweep them twice daily.My soil is adobe clay, not sand...BUT, we added crusher fines to the runs years ago, so there IS a kind of 'sand' where they live daily.

  2)I fed ONLY alfalfa for many, MANY years(I've been taking care of my own horses for nearly 60 years)-BUT, now I always feed some alfalfa, and some grass hay(never 'grown mixed'; I buy GOOD alfalfa and GOOD grass(and no one on earth is more 'picky' than I am about hay quality), and mix according to each's needs. (NM alfalfa has few if any issues w/ the blister beetles, thank goodness-plus, I drive a 240 mile ro. trip yearly to get my alfalfa from a PROVEN, and HONEST, producer, who I know is following proper growing procedures-fertilization and the like.)

3) I live where there is virtually never grazing available-high altitude, dry, only native grasses, if anything--and I don't believe in stripping off the vegetation, leaving only weeds and dirt to blow!)So, my horses have to get every bite from ME.

4) Feed PLENTY of good hay! I now feed three times daily, so that they don't go so many hours between meals...they just DON'T need empty stomachs for long; it leads to ulcers and worse. For a long time,  I resisted the extra work of it, but am glad I 'gave in', for my horses' sake.

5)I DON'T LIKE bermuda hay,and wouldn't feed it; the 'fineness' is almost certainly a factor in impactions. Thankfully, it has to be imported here(as does about every other grass hay, though some K-31 fescue is sometimes grown locally--but I don't care for it, either.) Too, everything I've read about using round bales(not common here, either) makes me certain I wouldn't use them, for several reasons.

6)Soaked beet pulp (yep, more work!) can add favorably to a horse's nutritional profile(in proper quantities), provides a beneficial kind of fiber, and is an excellent way to help keep horses hydrated, esp. in cold weather-and can also help w/ moving sand along and out. My horses get theirs at night, after their 'early' meal of hay.

7) Addition of reasonable amounts of loose salt can help; lots of horses may not take in enough by licking a block--and as stated, it helps encourage the drinking of water.

8)I fed psyllium for YEARS, since it was first promoted for horses. Even before that, I'd been advised by my local vet to try Metamucil on a mare who, shortly after I got her, passed a couple of manure piles that were essentially STRAIGHT GREY SAND!! Scared the heck out of me! She is still with me today, at age 27; she was not yet three when this occurred! However, I have been adding a stabilzed ground flax daily for awhile now, and am dropping the psyllium, as the flax, in the proper quantity,should provide the same benefits. BTW, you should NOT WET psyllium BEFORE it goes into the horse; you want that to happen INSIDE! When I've had little 'ground vaccumning' foals, I've had to use psyllium on them as young as a month or so; for that, my vet recommended a dose syringe and sugar free pancake syrup...messy as heck, but it worked!

  In the interests of full disclosure, I'll say that the horses I have now are all miniatures...but, I owned, trained, rode, used, showed 'fullsized' for more years than I care to acknowledge, and would (and did) do all these things when I still had them, too. It's just cheaper and easier w/ minis!! :)

Good luck w/ your mares; sounds like you already have a good handle on the issues!

Margo in NM (aka olehossgal)