COTH solved a very similar problem for me.
Herd of 3:
21yo Hackney Pony
7yo 34" mini
Mini near-foundered May of 2020.
Per vet I caught it very early (whew!)
Before that, he’d been on the same diet as the others for 4yrs:
Horses bring themselves in for hay & grain AM & PM, hay only ~10P.
Grain is whole oats & BOSS.
Mini got about 1 cup oats, maybe 1#.
Hay is 1st cutting orchard grass
Hay 3X daily, fed on the ground in their stalls.
Mini routinely hoovered his grain then went in with horse to “share” any grain left & hay.
Horse was okay with this
My setup is a lot like yours.
3 stalls that are open to pastures 24/7/365.
When grain is finished they go back to my far-from-lush pastures.
After mini’s laminitis, he was prescribed Thyro-L & grain became Triple Crown Sr.
I now lock him in his stall until horse finishes his grain.< COTH solution
I got a Tough 1 muzzle/halter combo that has stayed on.
He gets it on after morning grain & then joins horse for hay-sharing.
Muzzle comes off at PM feeding.
They are out on pasture overnight, but it is effectively a drylot this time of year
So he’s muzzled ~9h daily
When grass is gone & snow falls I’ll try him w/o the muzzle.
This has worked great for me.
He slimmed down - now fits neatly between the shafts of my cart.
Pre-founder, he was admittedly overweight, shafts pressed on his sides.
And shoer asked me if I was planning on butchering him
We just came back from The National Drive where we did daily 1-2h trail drives, a Cones Derby & 1h+ Driven Dressage lesson.
More driving in 2 days than he’s done in the last 2 months & he’s sound & fit.
After 3 muzzle-free days (no grazing @ the Drive) I put the muzzle on overnight last night. Afraid he’d gorge on pasture.
Muzzle went back on after AM grain today & came off before PM grain.
I’ll leave it off tonight.
It’s a Balancing Act for sure!