It depends who's growing the Orchard grass hay.
When I first moved south, where Orchard grass hay is the norm, my horses wouldn't eat it.
The hay I initially found was on a small farm and they were just looking to get rid of their excess that first year we lived here. It was a long way from quality hay to begin with, so most likely that's why my horses didn't want it
The next year, I had found a guy with brood mares who raised his own hay and sold the excess; top quality mixed grass hay. He still feeds my horses but he lost his first cut this year, so I found myself at the feed store looking at their hay to get me thru to the Grower's second cut.
The feed store got in a load of "professionally grown" orchard grass hay from the north end of my county. When you raise 6,000+ small squares for the purpose of selling, you're in the professional league - lollol
I've never seen or smelled such great hay since I've lived here. My horses don't waste one morsel of it. They ALMOST would rather stay on their hay instead of eating their supplements when I put their feed pans in front of them
Orchard grass is a good hay to feed if it's properly tended to before cutting/baling. I have two metabolic horses and it's also an asset for them.