guy,

maybe I am missing your point, but I think quite the opposite is true. You CAN grow 205 bu. corn with much less than 230#N depending on the year.

This plot tells us that to add 30#N, you would only get a 5 bu. (approx) yield bump.

My cost for N (make that sidedress NH3 as I do not have the 30# starter N cost/# in my head at the moment).....

.....my cost/#N was $0.437 this year. So that tells me it cost me $13+ to yield 5 bushels more. 

Last year The plot only gained 4 bu. from an extra 40#N applied. I don't have the costs for that right no either.

The take away is that I want to know where the sweet spot is, the Economic Optimum Rate of Return. Don't confuse this with Agronomic Optimum Rate.

I never know what the weather will do. I do know......

1)my yield history for my farm (5year 177+, 10 year 171+ BPA)
2)Cost of N (or at least a close guess)
3) Presently I generally get the best Economic return with .8-.85#N applied per bushel corn grown

With these I can run scenarios inputing different price ranges in for corn to find varying Economic returns.

I usually run about 140-150 #'s N applied. Last year knowing my N cost through prepay, and that corn prices were rising I bumped my applied N at sidedress time 10#'s /acre. I also had broad cast 50#'s AMS the fall before for about 11#N. An extra 20#'s N from normal. Yet the rising price/ bu raised the Economic return higher up the Agronomic yield curve.

The extra N paid for itself with interest "this year".

Disclaimer: The numbers in this post are for my farm which are not to be co-mingled with the neighbor's field in which the N-rate plot was in this year.