Silage covers are used to prevent air ingress into the pile, and by extension, out of it. However, that merely contains the odor. Once a silage clamp has been opened, the odor (or fragrance) of the silage will inevitably escape. However, again, silage is not meant to be left open and exposed to air. Properly closed, silage odors should dissipate at fairly short distances from the pile.
If you're troubled by the odor of silage, it could be a problem of poorly ensiled or managed fodder. Poorly ensiled material may harbor colonies of clostridial bacteria which can smell rancid, putrid, or fecal. The ultimate solution to managing this kind of odor is to improve management of the ensiling and feedout processes.
