Fred's right about safety, safety has little to do with the RCD. Plenty such installations have run reliably and safely with no rcd. The fact that a 30mA does not protect livestock is not a significant issue, there is already plenty of safety protection in place.
I would beware of using a local earth rod, as that can under fault conditions lead to ground voltage gradients that kill livestock if stood close to the rod.
A 10mA rcd would clear any leakage fault quickly.
NT