Just a guess but sounds like moldy hay.
Do you think just maybe it was bad hay or maybe rye or wheat grass bloat? It IS about that time of year.
I see some rancher looking for a payday.
My brother let six momma cows get into the range pellet feeder just free choice. They all died. Looked like pulmonary edema as well and a whole lot of bloat. Dug a big hole and buried them but we had a rain and their legs started to surface so he went out and cut them off with a chain saw. Really. I still laugh about that.