During the past 70 years, hybrid corn varieties have increased both yield and nitrogen use efficiency at nearly the same pace, largely by preserving leaf function during grain filling. The Purdue University study's findings offer strategies for corn breeders who want to continue to improve yields and nutrient efficiencies.
source https://www.lifetechnology.com/blogs/life-technology-news-blog/study-highlights-nitrogen-efficiency-gains-in-corn-hybrids-over-70-years