Facing the single strand of string, cut the string just where it runs past the bag.
My wife and I get chicken feed from several places in town that carry different brands, and the format for opening the bags with string can be a little different on each one. If you’re lucky — and perhaps have paid a tad more for this convenience — there’s a strip of paper on one side of the bag that, when pulled, separates the woven string from the bag. It’s easy-peasy. I recall a few occasions where only a little bit of string hung off the side, and I could see plainly how to pull the strings apart to open the bag. Another very easy option.
Then there’s the not-so-easy bag. Of course, this is probably the most common feedbag. It has about a 5-inch bit of woven string hanging off each side of the bag like Pippi Longstocking’s braids.