Call me simple, but I'd just dig next to the first waterer, and find out what direction the supply line is pointing. It's not like the prior owner would be pulling pipe in S-curves.
As to using a tone/tracer, yes it can be done with plastic pipe. You just need to back fill with something conductive (eg. salt water), and connect your tone source to something metal in contact with the water (eg. valve). Attenuation will likely be too high to trace it very far, but again... they're probably run in nearly-straight lines.
So, I'm supposing you can't talk to the prior owner? What about a neighbor, who might have been friendly enough to know the property, or a prior employee of the farm?