This is the toughest of our three 50kg bags because it's built as two separate layers doing two separate jobs. The outer layer is woven polypropylene — the same tough, tear-resistant material used in heavy-duty sacks and feed bags — which handles the physical abuse of being dragged, stacked, or set down on rocky or stubble-covered ground. Inside that, a separate plastic liner does the actual airtight sealing job that keeps the fodder anaerobic, since woven PP on its own isn't airtight.
That two-layer approach costs a bit more to manufacture than a single-layer LDPE or black bag, but it's worth it specifically if your storage site has sharp stalks, rocky ground, or general rough handling that would puncture a single-layer bag. If your storage site is a flat, cleared area with careful handling, the LDPE or Black options do the same fermentation job for less.