As several others mentioned, a 4 wire system is a good thing, but neutrals and grounds should not be bonded at subpanels. Someone mentioned a grounding plate electrode in place of a rod, so I looked it up. It must be a conductive material (duh!) and must have a minimum surface area of 2 sq ft in contact with the gound. If made of steel it must be 1/4" thick. If made of non-ferrous metal it must be a minimum .006" however aluminum is not permitted. Here's the catch, it must be buried at least 30" deep to be code compliant, so if you have rock right at the surface it may not help! (all info paraphrased into english from NEC Article 250.52 "Grounding Electrodes"
John T also got me curious as to special ag building requirements, so I perused Article 547 just a little also. For the most part, the special ag requirements are what one would expect as far as protecting equipment and wiring from damage by livestock, corrosive environments, and water, but a couple of things popped out at me that I was previously unaware of. One is that any electrical system in a livestock area must be equipped with a "site-isolating device." By the NEC definition, a site-isolating device is just a disconnecting means (disconnect switch, main circuit breaker, pole-top knife switch with ground operated handle, etc) It struck me as odd because I don't recall the term "site isolating device" being used elsewhere in the code. If your cattle shed panel has a main breaker, I would say that qualifies. The second was a provision requiring equipment grounding conductors to be copper, and if installed underground it shall be insulated copper.
The third item that I noticed in Atricle 547 involves equipotential planes, which someone else also mentioned. Basically, in concrete floors, in all areas where "metalllic equipment is located that may become energized" a grounded grid must be placed in the concrete. I think a waterer with connected heater would qualify as one of these locations. This is simply a mat of rebar or conductive mesh that is placed below the concrete and bonded to all metal structures and connected to the electrical grounding system. The idea is to prevent the stove-to-sink type shock mentioned in another post. If you have ground rods driven at all waterers, that is definetely a good thing, but make sure that they are connected not only to the equipment grounding conductor, but to the case of the waterer as well.
As far as splitting your bonded neutral panel into an unbonded panel, you can buy ground bars at any hardware store. With all wires disconnected from the old neutral/ground bar, place an ohmmeter between the bar and the ground rod wire, as well as the bar and the panel case. If it is properly isolated, it should have an infinite resistance. Then you can reland all neutral conductors on the existing neutral bar and move all ground wires including the one coming in with the feeders to the new ground bar which gets bolted directly to the panel case. (there are often pretapped holes in the case for this purpose)
I know this got a little long, but hopefully it was helpful...