We feed tons and tons of bluegrass straw to our cattle. It is a filler.
I have never seen a bluegrass crop salvaged as hay instead of being harvested for the seed crop. I am sure it happens, though.
Bluegrass straw varies widely in quality. If you run the hay baler right behind the combine, you can get some nice, palatable stuff. If the straw sits a while, it can get yucky.
Some years, I have it for my horses to eat as a filler, our orchard/timothy hay is super premium, high quality stuff and the horses don’t need a lot to meet their nutritional needs, so they do want to chew on something. But some years, they eat it and some years, they don’t. If there is a bit of green, and no moldy smell…they MIGHT like it. If it’s yellowy and moldy, they for sure won’t like it.
As far as other grass seed crop residue, I know that feeding ryegrass straw is a huge risk because of the ‘fescue toxicity’ issue. A rancher in Eastern Oregon lost a lot of cows when their hooves fell off, from the ergot in the ryegrass straw.
So, you could watch Central Oregon craigslist in late July or August, when they harvest grass straw. There are a few people putting it up well, in small bales, mostly in the Madras area.