It's not a matter of animal waste getting on the raceways or cables, it's a matter of the vapors caused by the waste material. Those vapors can be highly corrosive. The other consideration is some of the corrosive chemicals that can be stored or used in Ag buildings.
MC cable, if used, must be of the jacketed type. The cable armor isn't sufficient in these instances. That having been said, 547.5(A) leaves quite a loophole with the phrase "...or other cables or raceways suitable for the location, with approved termination fittings..." So, a lot of this is really going to depend on what the specific use of the building will be. Best to check with the AHJ to see what he requires. Not many Ag buildings won't fall into either the Excessive Dust or Corrosive Atmosphere categories.
It's also best to give 547 a good going through. Lots of requirements there that might bite you if you're not familiar with them. And, again, a lot of it depends on what the structure will be used for.
