Hay is often bound up into bales with plastic twine. Cattle happily eat bits on accident. They used to use wire, but that caused a much more serious problem for the cattle.
They used to use natural twines like jute, which was better. They really don't eat much of the plastic stuff. The larger pieces of it do kill them (choking, cholic, etc).
Silage uses insane amounts of plastic wrap and then is left in direct sunlight to decay