Came here to say this. Supps or feed outside the stall would be a No Go at every barn Iāve ever been at. Thereās barely room for blankets and fly spray!
Iām sure some places go down the aisle dumping supps like that, but everywhere Iāve been kept ALL feedstuffs away from the horses in the feed room. Often just one person sets all the feed ahead of time, and feeders just go down the aisle dumping buckets.
This is why itās best to just talk to the people who run the place, preferably the actual feeders. I prefer cups or baggies and zero liquid supplements, but Iāve been the person doing feed for large barns enough to have an idea of what is easy and what is just a pain. It may āonlyā take you a few minutes and some brain power for your one horse, but multiply that by 40 and you can see why someone might have preferences!