Hey T.J.!
Membership in the American Topical Association might be worthwhile if you plan to stay with the interest. They have a list of topics with worldwide "checklists" here:
http://www.americantopicalassn.org/...cheklist.pdf -- I don't see a "chickens" separately, but there is "birds" at 131 pages! Might be a lot if you have to pay by the page. I don't see a separate "farming" category.
If you do want to make it a worldwide collection, I can probably help with putting together a non-exhaustive list (chickens will be a much shorter list than farming.) Let me know if that's the sort of help you'd like.
Most importantly, don't let anyone tell you what or how to collect! There are a lot of good ideas out there, and very important information on how not to damage your stamps, but in terms of what stamps you want to collect? The world is your oyster. (And there are oyster stamps out there, too!)