This – but it really, really depends. Your contract will have the answer, and if it doesn’t have the answer, you need to bring that up with the manager and ask them why they are charging you XYZ when it is not written in the contract.
In the scheme of things, 6lb for an UL horse is not that much… Mine almost gets that and he isn’t even in work… Around here, in just about every eventing barn I’ve been in or worked out of, 6lb a day is not considered a lot of grain for an event horse in work… most UL eventers get north of 12lb a day, usually of oats, alfalfa + their preferred grain… 6lb is kind of pocket change in the theme of things…
But, if this is a western barn or a barn with stock horses, which by nature seem to need less to maintain good weight, 6lb seems like a hell of a lot when Daisy and Skipper, the two cow cutters in the stall across, only get a can full of oats…
However, I would look into trying a different grain before feeding more - if your horse is only barely keeping weight on, the barn is not doing their job; they should be increasing hay first before grain, and you should look into seeing that he has hay in front of him 24/7, especially if they are going to be charging you more $$ for grain.