Purdue University in their study emphasized that over the past 70 years,
yield and nitrogen efficiency of hybrid corn varieties have increased
significantly. The study's purpose is to identify how corn plant utilized
nitrogen throughout the years to guide breeders in their decision making
involving future hybrids. According to Tony Vyn, a professor at Purdue's
Department of Agronomy, they observed a "progressive improvement" in the
nitrogen use efficiency in corn hybrids, which then results in higher yields
as modern hybrids are able to absorb more of the nitrogen fertilizer
applied.

Results highlight that genetic improvements in corn hybrids have led to an
89 percent increase in grain yields and a 73 percent increase in nitrogen
use efficiency from early hybrids up to the present. Having less nitrogen
fertilizer not absorbed and wasted, the grain created from the absorbed
nitrogen increased from 42 pounds of grain per pound of nitrogen taken up in
the plant to 65 pounds of grain. Moreover, suggestions for breeders about
how to continue improvements in yield and nitrogen use efficiency were also
included in the study.

[purdue.edu]
iency-gains-in-corn-hybrids-over-70-years.html