Ok , we put plastic on top as well( thought everyone would do that) and trow 20 cm of sand on it, no loss at all but you are right, in the winter its difficult, our winters ain't that tough.
Over here we would have put plastic on the sides so you would not lose good feed because of rot.
Yields around here go from 35 to 60 tons per hectare with about 32 % dry matter.
Harvest is just about to start now and prices per hectare are €1500-1800.