Are agricultural pilots allowed to use a highway as a runway?
My answer is colored by having spent a couple of years as an ag applicator.

There is no endorsement needed to use the highway as a runway.

Practically, it is done less now than 30 years ago.  Also more states have codes now which prohibit operating aircraft off public highways.  

Locally an operator used a paved road which terminated at an interstate, where all the houses on that road were owned by the same family. In 1978 that operator had to scramble, because the town decided to enforce a code prohibiting aircraft from using town roads for landing and takeoff.  The only other place I have seen it today is in parts of the northern mid-West. I am not familiar with their local codes or state codes.