Back then, they used feedsacks as a marketing tool for fabric sellers. They kept track of the prints that were selling well, and which ones didn't. The prints that sold well were reproduced into fabric yardage. Pure genius, right?
(The average woman needed 3 sacks of the same print to make a dress for herself. When the feed companies started packaging their products in these type of bags, the women were seen more and more in town shopping. I guess they were afraid their husbands would pick up an ugly bag. lol)
As redmadder said, look for the stitching holes in it to determine if it is really from a feedsack or not. The print might be exactly like one found on a feedsack, but it might not be something that once held flour, corn, or salt, etc. (That would not be in a yardage.)