The resistance is unimportant if there is no current. Whether current exists or not will depend on how strong the voltage difference remains. It may be the equivalent of reading an induced voltage with a digital meter, but not reading a voltage with an analog meter.
That would be correct, but in general that type of voltage cannot be felt by the animal.
Driving a ground rod is a lot cheaper than installing an equipotential grid. I would drive the rod to begin with, but make sure the customer is aware that there is a chance it may not fully correct the problem.
I think there is a lot more than a chance that it will not correct the problem. In my opinion there is only a small chance that it will correct the problem.
Even if it is a hard voltage difference (with current flow) it will still reduce the voltage sensed by the horses because there will be a voltage drop across the ground wire. Assuming that the power cable is quite long, voltage division between the earth and the ground wire will have a significant effect. If the horses can still sense the voltage, driving a second rod at the opposite end of the trough is still a lot cheaper than installing a grid.
In most cases this voltage is from the voltage drop on the utility primary grounded conductor. A ground rod will not solve this problem. The ground rod will never flow enough current to cause a voltage drop that will reduce the voltage felt by the animal.
It will reduce the voltage for a small area around the rod, but given the size of the horse, it won't really help. At ~3' from the rod you will have ~85% of the original voltage. Even the second rod will not likely help. It is still my opinion that the only effective corrections are, neutral blockers, complete system isolation or a equal potential grid.
