I agree that yes the cows do have a reference to the electrical system.
However that would only be under fault conditions.
The biggest issue I've ever seen with voltage leakage in a dairy shed was a couple of years ago I got sent to diagnose a fault, where the workers claimed they were recieving shocks off of the platform and the rails around the platform.
Please bear in mind that I am talking about a rotating platform here, not a standard herringbone pad type shed.
Considering that the workers in any dairy shed have wet hands (and sometimes even get wet clothes)this is rather a worry, because to a certain degree the rails are at Earth potential, through the wet concrete.
I tried to find out where the voltage was coming from, but it wasn't until I got the farmer to start the platform up that I really started getting some bad readings.
I managed to trace the problem to the sliding contacts at the top of the platform cage (above the middle of the centre of the platform, where water, power and milk run through, as the platform rotates).
The Earth conductor had corroded through and at the same time, one of the sliding contacts (almost like a motor brush on a slip-ring motor) had become mis-aligned from its usual phase ring and was dragging on the Earth ring.
Anyhow it turned out the firm that installed the electrical gear had used non-standard parts and had substituted some of their own.
I'm beginning to think that this is a situation which isn't covered by regulations, and maybe it should be.
As far as I know, AS/NZS 3000, is the document of choice these days and any part of that could be applied to any part of any installation.
I would however have a read of the Standard for the connection of Generators to Installations (Of the actual Name and number of that Standard, just escapes me).
We lost a whole heap of well written and informative Electrical Codes of Practice (ECP's) here not long ago, that took good money to write and implement, now it seems we must buy new ones at twice the price and with half the information.
Since I've recently cancelled my Practising Licences, I'm no longer eligible to download the Standards in PDF form.