I love watching horses use their practically prehensile lip for sorting. I have considered taping it, finding appropriate sound track & loading to you tube. It kills me.
My mare really works hard to get every possible piece of chaff from hay. I have seen her do this after the other horses have given up & moved on: she is still nosing around what looks to me like bare ground for a long time. When she is outside it looks like she doesn't get any dirt. I have checked for sand in her manure.
Anyway, she is stabled inside at night this winter (first time for her) & tonight I was watching her doing her thing, and I can't make out if she is getting shavings in her mouth or really sorting out the yummy bits. I tried to check her mouth for contents but still couldn't tell.
So how good are they, really, at sorting? If she is getting shavings, then what? I can't imagine a way to prevent it.
Well if it's any consolation, I have a gelding that will just eat shavings. I don't mean lip through them. No, he gets entire mouthfulls and chomps them down. Before anyone says he isn't getting enough forage, he does this with a full hay net of hay right in front of him. I have no idea why he does this but it doesn't seem to cause him any problems. I will admit though he is quite possibly one of the weirdest horses I have EVER met.
"Be the change you want to see in the world."
My gelding used to be in a paddock with a manure pile, with the conveyor belt running out to the pile from the barn. He and his buddies liked to stand under the belt with their heads up to catch any little stray bits of hay in the air that dropped down from it (which also ended up in his mane and all over his neck along with the manure). (He wasn't lacking for hay and grass, but this was a fun way to pass the time).
He also liked to climb 15 feet or so up the pile to pick hay bits from it. One of the funniest sights he has ever treated me to was seeing him standing way up in the pile and all his buddies standing below and watching him but not being brave enough to climb the pile, too.
My mare will also EAT shavings. I discovered it this summer when the barn manager at my previous barn insisted that she come in during the day even though she was on pasture board so that they could conserve the small pastures. She was wearing a muzzle 24/7. He'd bring her in for 4 or 5 hours without hay and she was filling her belly with shavings. I asked for hay, they didnt have any and didnt have plans to buy any until winter (wacky place) so I had to put my foot down and say she was not allowed in the barn unless they were going to take the shavings OUT of the stall because my vet said she could get an impaction.
If your mare is only getting a tiny bit of shavings, then its probably ok. I know my mare will not touch actual sawdust. She liked the taste of the shavings much more. Oh, and straw? Haha. Yeah, she will mow through straw like its hay.
Are you having to replace the shavings a lot? You can kind of monitor the amount she may be eating that way. Or, try to sweep the shavings away from your normal hay spot so she doesnt track hay into the shavings and have to sort as much. Maybe a slow feeder to reduce the amout of time she has to dig around. You can also muzzle her if you have one. They can still eat hay through a muzzle but it will take a long time.
The dopey gelding I've got at home for stall rest grabbed a HUGE mouthful of bedding pellets and started chewing them the other day, then spat them out and looked offended. A few shavings are not a problem--it's like eating straw or other woody-stemmed bits of forage. They can't digest them, but it's vegetable matter and assuming it isn't coated with chemicals or ingested in huge amounts it's not a big deal.