Historical trends in the nitrogen requirement of corn over 45 years in Ohio

The average annual corn (Zea mays L.) grain yield in Ohio has increased
since the 1970s, yet the respective roles of corn hybrids and optimal
nitrogen (N) fertilization in contributing to this historical trend remain
unclear. This study evaluated trends in the agronomically optimal nitrogen
rate (AONR) and corn grain yield at agronomically optimal nitrogen rate
(YAONR) when corn followed corn (CC) or soybean [Glycine max (L.) Merr.]
(SC) in the crop rotation within two eras of corn hybrid development. The
two eras were associated with different technological development phases,
including 1976–1995 (pre-transgenic era) and 1996–2021 (transgenic era). A
total of 431 rainfed corn fertilizer N rate response trials were conducted
in 31 Ohio counties over 45 years. From 1976 to 2021, AONR did not
significantly increase, while YAONR increased by 96.1 kg per ha per year,
a 52% increase over 45 years. Unfertilized (0 kg N per ha) corn yield
gains were similar to annual yield gain at AONR, and the agronomic
nitrogen-use efficiency (ANUE) greatly improved over 45 years, with an
additional 4.6 kg of grain per kg of N for corn grown in 2021 compared to
1976. Overall, our study demonstrated that historical yield gains were
largely due to improved corn hybrid ANUE and management rather than
changes in N fertilizer requirements.