I attended a Beef cattle symposium back in Oct and one of the speakers was Dr. Robert Lane of Sam Houston University. Forage and Legumes guy. Pretty informal get together and when he said "For goodness sakes, cover your hay--don't leave it out in the open. If you don't have barns enough at least tarp it."
Immediately, someone in the room said "If you tarp hay in East Texas, it will mold".
Doc said "Yes it will, and if you have that moldy hay tested, you will find the protein levels increased".
Now, I don't know about that, and I guess he knows what he's speaking about, but I sure wouldn't want to feed moldy hay just to get a little bit more protein. He did mention in his introduction that he is a beef cattle producer as well as being a Proff at SHS, so I guess he has some practical experience too.
I did tarp 8 bales this year that I didn't have room for under roof, and sure enough, it molded even tho I pulled the tarps every time we had a stretch of sunny dry weather. I'll never tarp any more. Ever.