We have 23 Shetland sheep on a 35 acre heather moor / heath that is left every summer to grow, so mine have lots of forageing, two troughs piled high of bb silage every morning ,four sections of hay everyday and a mineral lick. We dont feed bagged yet and they are thriving and have been commented on by top sheep men how good they look, so I guess the quality is good . We have a source for grass clippings so we can fill the barrels up with fresh grass quickly and seal , another key to good bb silage, it is no good left to fill over a few weeks Good luck , have a go. ps I do know on another forum someone does it in thick black bin liners , dump bags are porous I would think but you can experiment and report back. That is how most folk learn new things as indeed we have done.