Our feed store will take the plastic bags back and reuse them as long as you don't tear them or rip them, so when I go to pick up feed, I take my old bags back to them.
Good idea about using them to put manure in for the neighborhood gardeners !
We get our feed in the woven plastic bags. I use them to protect small trees and shrubs over winter. What I do is set up a tomato cage over the planting, then pull a plastic feed sack over the cage and wire it down with plastic wire ties. It helps the small trees get a good start if the wind and frost are kept off a bit.