It's light that fades fabric, and it actually does not take that many hours to do it. I think if you left your new feedsacks in a sunny window for several weeks, you would find them faded to a much softer look. If you Google, you can find info about how fast fabrics fade when subjected to light. After that, if the whites need to be yellowed, you could consider dying; however, I think RIT and similar dyes are preferable to tea dying as the tannins in tea are hard on fabric.