Interaction between motors, transformers, equipment and even other buildings on the farm. The modern farm has a lot of electrical equipment located in several buildings and exposed to a dirty, wet and corrosive environment. The electrical system has many of the same problems as an industrial plant with the additional problem of animals being susceptible to low levels of electrical shock. A farm's electrical system must be designed to compensate for all of environmental problems in addition to the goofy way the utility wires its system. Most farmers under estimate the importance of a well designed electrical system.
All farms are different and there is no generic design that applies to all.
It should be noted that the code is a minimum requirement and compliance with the code gives no assurance that you will not be subjected to stray voltage. There are many expensive solutions being promoted as magic bullets to eliminate stray voltage. Some of these schemes have varying degrees of success on specific instances of stray voltage. However, the best and most cost effective solution is to retain a professional engineer that knows how to build a robust electrical system that is capable of preventing stray voltage. Then, make sure that the system is not compromised by introducing equipment that can cause ground currents.