I give our junior silver fox and flemish giants almost a handful of barley a night and a teaspoon of Oxygen pellets. But we have issues with size especially the flemmies so we're adding the barley to put on bulk.
Barley is every bit as good for rabbits as wheat or oats. Some people feel it gives the meat a better flavour. The only reason I seldom use it is that it is not as easily digested by poultry and I prefer a grain I can feed to all the critters.
When you tried sprouting it, did you add a couple glugs of bleach to your soaking water(soak for 12 hours in it)? That made a world of difference for mine. I can’t seem to get my wheat to behave till winter, but the barley does wonderfully even in the summer that way. I had to switch them to the green trays (with more ventilation) for the summer, but in the winter I’ll go back to the big black trays that hold more.