I see on wikipedia reference a 1978 tractorcade to Washington DC but it goes right on to say 1979.

I see a little imagination working in a post above, I guess it makes a good story even if the month and year is not right.      lol

Here is a 2009 NAT thread where the tractorcade was discussed briefly w/a link to a video I did not look at because this laptop does not do sound:

 http://talk.newagtalk.com/forums/thread-view.asp?tid=79397&mid=568914#M568914  

It seems like NAT had at least one other thread, possibly with pictures but I did not see it with my quick search.

I was helping Dad farm in any "spare time" while managing the local co-op in the years leading up to the tractorcade.  They were turbulent times.  Many local local farmers lost their farms.