You mean the U.S. Department of Transportation? or
the state department of transportation?

Certainly the U.S. department has no jurisdiction
to regulate boat trailers or what you haul in the
boat on a trailer. (think interstate
transportation). The state does have regulations
on trailers, most individuals probably have no
idea what may be prohibited from being hauled or
the conditions required and couldn't care less. I
have my doubts that the state patrol has any idea
either and would stop a trailer only if it is
swaying. They wouldn't pay any attention to a barrel.

Li wrote:
You would also have to make sure the barrel was placarded or labeled. The
DOT has rules about anything like pulled over the road.