I'm going to come at this slightly differently. Though touch on the same points.
One) is the type of chicken farm raising birds for the same general purpose you are? It sounds like they are involved in commercial broiler production, likely Cx). Are you also raising Cx??? CX feed is designed for least cost production of meat for supermarket, with a high fat level. There is NO focus on long term health.
If you are raising layers, that's the wrong feed - Calcium is too low (which you can compensate for by offering oyster shell), protein is too high (which is simply wasteful and economically costly), and its a LOT of fat, particularly if you don't free range your birds.
Two) If their biosecurity is worse than yours, by routinely going there for feed, you are making your biosecurity no better than theirs. If their biosecurity is better than yours, why are they allowing you on premises to buy feed?
Three) If you aren't raising supermarket meat birds, the best thing you can do is have a pleasant conversation with them at the mail box, find out where they are getting their feed. Assuming its not delivered to them under contract from Tyson, ConAgra, Wayne, Purdue, etc then there is likely a local mill producing reasonably priced feed appropriate to the types of birds you ARE raising. If it is delivered to them under contract, and they are selling same, that's not the sort of folk you probably want to associate with.
and you can get your local Mill answer as easily from your local Ag extension office [phone call], or by visiting the local co-op (change your clothes including shoes after, before entering your coop area - you don't know what their biosecurity is like).
Hope that helps?