If they are capable of eating double that, why not double their hay ration? Here we don't ration hay at all- it's available free choice. I've never had an issue with anyone being overconditioned on just grass hay. Just big healthy hay bellies. Brome is not known for being high in protein like the legume hays, so I wouldn't think free choice would be a problem.
Because if I don't they get fat, not just big bellies but actual fat! Took me a long time to figure out why they were so fat and wouldn't lose any weight!
All summer long this year they are getting all the browse they want plus hay and the ones that don't need to gain weight aren't so I'm pleased.
I think a friend told me that it's possible that for some reason that particular hay (from that haymaker) may be high in sugar.