She's got a salt block, gets a complete feed (Horsemans Edge), has 24-hour access to nice green pasture and gets hay every day.

Why would she lick clay? Do I need to worry about this? I could close off that pasture if need be.

You could call vet and ask about a mineral test, see what he advises.I don't know what those tests run. We only test for selenium an essential element. None in the ground or local feeds, some horses need more than others. We test to make sure our horses are using selenium we provide, or need a different dosage. Age, growth or body changes can sometimes require more.

With complete feeds, most people don't feed the recommended amount, it is WAY TOO MUCH for the horse in light work. So horse does not get full dose. Perhaps a diet change of plain oats for snack with a seperate supplement added on top. NOT both top dressed and prepared feed! Get your horse messed up with too much supplementing.

Depending on what pond usually had in it, I might take her out of that field. If pond is the drainage point for all the local runoff, she could be getting fertilizers, other poor food choices! I would not be comfortable with mine licking mud, our field with the swale hole is closed right now also. As far as I know it only has wild animals visit, not any field drainage there. Just things exposed which are usually covered by water may not be good for my horses.

Velvet

This reminds me, one of my weanlings keeps licking the poultice off the other weanlings' ankles. He's been doing it off and on for about a week (and they do have salt & minerals in their field). Maybe he is craving mothers' milk still and poultice is the next best thing? Who knows, but it hasn't killed him yet.

One question about the mineral blocks... she's pastured with sheep, and I don't think I can use a medicated/red salt block with sheep. I could probably mount a small medicated block in her stall, too high for the sheep to reach, though. I'll try to pick one up on the way home from work tonight.

The people we bought our sheep from said the soil in our area is selenium-deficient, maybe that's what she needs.

Re: the pond, it's spring-fed and very clear, and doesn't get much (if any) run-off from the surrounding fields because it's actually at a slightly higher elevation than most of them (we're on the side of a hill). I'm going to move her off of that pasture tonight, though, when I get home.

I was hoping to avoid feeding supplements if at all possible,. Seems hideously expensive and potentially dangerous. Or am I old-fashioned??

If you are in a selenium deficient area, you need to be adding selenium and Vit E(helper allowing horse to absorb selenium) to horse diet. You can buy both items at the local elevator. They are not mixed evenly, check with your vet for proportions. I find them fairly inexpensive, easy to feed in powder form. Not a horse special product, so way cheaper than a name-brand horse item.
Horse needs selenium for muscle and reproductive tract. Being deficient can make horse very muscle sore, lead to poor heat cycles, sore backs, among other things.

I also thought my good pasture, nice hay, clean grain was enough. Getting a lame horse, too sore to use from being selenium deficient changed my viewpoint. Dr. doing the mineral test had never seen one test so low and still be moving. He came right back out to give her a helping shot, then told us to get the powder. Amazing the change that daily tablespoon of mixed powders made to the horse. My case is extreme. My other horse, same field, diet, showed no deficiency. Horses had been together for 10 years. Just the way two different bodies use what is avalilable to them.

If you have sheep, do they have a sheep mineral block? There is stuff in sheep feed that is bad for horses. The same way that copper in horse feed is bad for sheep. Horse could be getting wrong minerals from sheep feed or block, throwing her system off.
This is where over-supplementing can throw body system off. Horse only needs TINY amounts of these needed minerals. You have to look at all the stuff being fed, read labels, not add a little of this and that. Complete feeds are just that, COMPLETE, if fed at the rates maker recommends. Horse shouldn't then be getting extra, other supplemental stuff.

I'd never used mineral blocks of any kind before (other than straight salt). Last night I bought two Dumore blocks, one each of the horse and sheep variety. Both contain selenium.

Figuring out how to keep them from eating each other's blocks was the hard part, I puzzled over that for awhile.

I ended up putting the sheep block in their stall (horse can't get in, door is too small).

I stuck the horse block into an unused-feed bin in the horse stall (sheep can't reach it, they're too small and fat).

I also closed off the gate to the pond, and got a fresh plain salt block.

Hopefully it'll do the trick.

Our horses just were not getting sufficient selenium from mineral blocks. We had to give it individually with supplement. They had the mineral blocks when the one horse went lame. Might be how we use our horses, they work hard, sweat, when we are conditioning, competing them. The muscle activity might demand a higher selenium need than a pasture horse or one used lightly.

I hope this takes care of the dirt licking for your horse.