Here the recommendation is to do last N just before tassel.  Right or wrong, not sure.  The pivot was stuck.  I mean stuck.  It would have taken 2 weeks for it to dry out, if it did not rain.  We need to be able to make additional irrigations later in the year 99% of the time.  So you drive in the tractor, tower #2 and not far from the pivot motor service road and pull out the pivot, but I could tell the ground was just mush ahead of the pivot so we needed to pull the pivot an addition 200 ft or so to get onto firm ground and then it could pull it self.  Now we need to go back in and put in rock so we can do additional irrigation this year.  When it gets hot you make a round with the pivot every 7 days and even then you are loosing ground on your ground water supply to the corn.  It is a slow loss and you hope it rains, but it is not uncommon to go 6-7 weeks with no rain and 95-100 temp with strong south winds of 20-30 mph day after day.