Am I required to register my farm if I sell grain stored on my farm to a neighboring farm, and the grain could be used by the neighboring farm as animal food?

No. A farm that grows grain, stores it on the farm, and then sells that grain to a neighboring farm
or another person is acting within the definition of “farm” in 21 CFR 1.227. The farm would still
be exempt from registration even if the neighboring farm or other person uses the grain as human
or animal food at a later time.