Dr. Franzen discusses this topic. keep in fertility is local but still good information!
It blows my mind how many guys don't track their N efficiency. Dr Haney has done a lot of research on the subject.
Some organic guys I know really focus on soil biology and are raising ~180bu corn with zero applied N. According to a Becks agronomist a couple weeks ago if soil is functioning properly every 1k plants should be 10bu with zero applied N.
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