This issue has gotten lots of attention around here in the past year or so, so we had a DOT officer speak at our county cattlemans association conference. I'll try to recap what he said...
1) If the gvwr of your truck plus the combined gawr's of your trailer (because the trailers gvwr includes tongue weight) is 26,001 # or greater, then you need to have a cdl, dot numbers for your truck and trailer, and all that other fun stuff
2) If said combined weight rating is 26,000# or less then you (including your family and employees) are legal within 150 miles of any agricultural land you own or lease, with a standard drivers licensce and farm tags
3) If the combined weight rating is 10,000# or less, you are under no restrictions (even a half ton pickup and a smallish trailer will most likely exceed that weight rating)
4) If it's for recreational activities, then the dot restrictions don't apply. According to this officer, livestock shows qualify as recreational, if you're not going for the purpose of selling the animals - this can be proven by official health certificates that state the animal(s) are being transported for exhibition ... (not sure if that's "official" or just kind of a rule of thumb that they use)
5) He also said that unless you have attracted their attention otherwise (moving violations, non working lights, etc...) that they will very rarely question a single rear wheel truck pulling a trailer with tandem singles, maybe a dual rear wheel truck pulling tandem singles if something looks iffy, but will pretty much always check out anyone pulling a trailer with triple axles or tandem duals - none of that's official, of course, but just some things he mentioned that they look out for
I guess it just goes to show how much difference there is in how the laws are translated and enforced from place to place. Too bad they can't be more uniform, but at least I'm glad they seem to be pretty easy to get along with here compared to some other places.