john barleycorn has nothing to do with smoking marijuana

Agreed. Does the Traffic version say anything that implies this theory given above, i.e. they are advocating consumption of 'pot' instead of ale? I don't have the album so I can't check one way or the other.

(To answer a query above, Traffic was a 1960s band formed whose members included Steve Winwood, Dave Mason and Jim Capaldi. They had a few hits in the UK including Hole in my Shoe, and Paper Sun, and their music was what was called psychedelic in those days)

Yes, there are lots of different versions and tunes as with many old songs.