Would RFID work for this particular application? And would it be sufficient for a very large field?
RFID is a relatively short-range technology. When it is used for animal tracking, readers are typically set up at the gates through which animals pass. This allows users to know, for instance, that a given cow was in a particular field at a specific time, and not in a barn. RFID cannot tell you where an animal is in a very large field, however, as it does not have that kind of read range. I would suggest utilizing a low-cost GPS device, and that you check out Iota.
—Mark Roberti, Founder and Editor, RFID Journal