The next step on my critical path is evaluating the ESP8266 deep sleep mode. So to that end I have ordered an ESP-12 so I am waiting for that to arrive now.

For back to farm communication I am also researching the use of 433/915 MHz ISM band modules, which with the right antenna I should get 5-10-20km line of sight. It would be handy to ave a system that doesn't require the phone network. I think with the right controller setup in the fence posts, they can all act as store and forward / redundancy - it wont be a mesh - and I'm informed the ISM band comms works best over point to point.