Corn Response to Reduced Nitrogen Environments in a 17-Year Study

• Beginning in 2006, Pioneer conducted an annual study to evaluate the response of corn in limited nitrogen environments.
• This study was unique in that each nitrogen rate treatment was positioned on precisely the same field area each year.
• This allowed researchers to examine corn response to each nitrogen level over multiple years of production.
• In addition, after many years of no added nitrogen, the “zero-N” treatment represented a truly nitrogen deficient environment.

• The reduced nitrogen study was conducted over multiple years between 2006 and 2014 at four Corteva agriscience research stations located in Corn Belt states (Table 1).
• The study at the Johnston, IA research station was maintained continuously up through 2022.
• The York, NE location was irrigated; all others were rain-fed.

  Table 1. Reduced nitrogen study locations.

  Study Locations	Years
  Johnston, IA	2006-2022a
  Windfall, IN	2007-2014b
  Champaign, IL	2007-2014
  York, NE	2008-2014

  ^aYield data not collected in 2012 (extreme drought), 2015 (wind damage), 2018 (flooding and wind damage), 2019 (flooding and wind damage), and 2020 (extreme wind damage).

  ^bYield data not collected in 2011 (flooding) and 2012 (extreme drought).

• Each location included two crop rotations and five nitrogen rates within each rotation for a total of 10 treatments:

  Crop Rotations:

  • Continuous corn
  • Corn-soybean rotation

  Nitrogen Rates (% of full rate):

  • 0%, 50%, 70%, 100%, 130%
• Each location included two corn-soybean rotation blocks, so that corn yield from the rotation could be measured every year of the study (Figure 2). One block was planted to corn in even-numbered years and the other in odd-numbered years.

• For the first six years of the study, nitrogen rate treatments were based on a percentage of a “full-rate” treatment.
• The nitrogen rate considered to be a full-rate (100%) treatment differed among years and locations based on determinations of economically optimum rates for each site-year.
• Beginning in 2012, the study was changed to use a fixed set of nitrogen rates that were maintained continually at the Johnston, IA location through 2022. Actual nitrogen rates for the Johnston study are shown in Table 2.

• Nitrogen treatments were applied as a single application. Application timing varied from immediately after planting to approximately V2 stage depending on the location and year.
• Nitrogen was surface applied as ammonium nitrate during the first four years of the study and as stabilized urea from 2010 onward at all locations except York, NE where it was sidedressed as 28% UAN (Figure 3).
• Nitrogen fertilizer was applied by hand to ensure precise placement (Figure 4).
• Each rotation x nitrogen rate treatment plot was 8 rows wide (30-inch row spacing) and length varied by location. At Johnston, the plots were 122 ft long from 2006 through 2018 and 105 ft long from 2019 onward.

• The corn hybrid used at a given location differed from year to year and was typically a Pioneer® brand corn leader product for that particular geography.
• In 2011, 2012, 2013, and 2014 the nitrogen rate treatments were split between two hybrids to determine whether the hybrids would respond differently to crop rotation and reduced nitrogen environments (Table 3).

All Locations: 2006-2014

• Across all study locations, corn rotated with soybean had greater top-end yield and less yield penalty associated with reduced nitrogen fertilizer rates compared to continuous corn (Figure 5 and Figure 6).
• Corn yield response to nitrogen rate differed between the rain-fed eastern sites (Johnston, Windfall, and Champaign) and the irrigated western site (York).
• The irrigated site had less of a yield penalty associated with reduced nitrogen rates compared to the rain-fed sites, particularly for rotated corn (Figure 6).
• In rotated corn, the average reduction in yield with zero nitrogen was 35% across the rain-fed sites, compared to only 14% at the irrigated site.
• In continuous corn, the average reduction in yield with zero nitrogen was 65% across the rain-fed sites, and 46% at the irrigated site.

Hybrid Comparisons: 2011-2014

• The design of this study, with reduced nitrogen rate treatments maintained in the same locations for multiple years, provided the opportunity to compare hybrid performance in established nitrogen stress environments.
• Hybrid comparisons were first conducted in 2011, after each study location had been established for at least three years, and continued through 2014.
• Each eight-row nitrogen rate treatment strip was split into four rows of each hybrid, with the same two hybrids used across all sites.
• In 2011, Pioneer® hybrid 33T57 and 34N42 were compared in the study, based on previous Pioneer research which showed 33T57 to be more tolerant than 34N42 of low residual soil N levels, and less likely to lose yield under N stress.

• Pioneer 33T57 was higher yielding than 33N42 across all nitrogen rates in both rotations (Figure 7).
• In the corn-soybean rotation, both hybrids responded similarly to nitrogen rate, with yield increasing linearly up to the highest nitrogen rate.
• In continuous corn, the two hybrids diverged in their performance at the zero nitrogen rate, with Pioneer 33T57 showing greater yield stability than 34N42 under extreme nitrogen deficiency.
• In 2012, the hybrid comparison was repeated, but with two different hybrids, Pioneer P1498AM™ and 33D53AM™ brand corn, with the former expected to show greater yield stability under nitrogen deficiency. This comparison was continued through 2014.
• Results differed between the eastern rain-fed sites and the western irrigated site.
• Across the eastern sites, yield response of the two hybrids to nitrogen rate was very similar in both rotated and continuous corn (Figure 8).
• At the irrigated site, P1498AM had a significant yield advantage over 33D53AM in continuous corn at the higher nitrogen rates.
• The hybrids responded similarly to nitrogen rate in rotated corn.

Johnston, IA Long-Term Results: 2006-2022

• The study was continued at the Johnston, IA site through 2022, providing the opportunity to look at long-term trends in corn yield response to reduced nitrogen environments.
• There were a few seasons in which the ability to collect quality yield data was compromised by severe weather damage, so results were grouped into two time periods: 2007-2011, which included an unbroken five-year stretch of yield data, and 2016-2022, which included four years of yield data from 2016, 2017, 2021, and 2022.
• During the 2007-2011 period, rotated corn had greater top-end yield and less yield penalty associated with reduced nitrogen rates compared to continuous corn (Figure 10).
• In the zero nitrogen treatment, yield was reduced by 60% in continuous corn and 37% in rotated corn.
• Yield of both rotated and continuous corn increased substantially in the 2016-2022 time period compared to the 2007-2011 period, reflecting the genetic gain and higher yield potential of newer hybrids (Figure 10 and Figure 11).
• Nitrogen treatment rates were higher for the latter time period, due to the switch to higher fixed rates beginning in 2012.
• Yield response to nitrogen rate in continuous corn was similar between time periods, with the zero nitrogen treatment yielding 60% less than the full nitrogen rate treatment in both 2007-2011 and 2016-2022.
• In rotated corn, however; nitrogen rate response changed considerably between time periods, with yield response to nitrogen nearly disappearing during the 2016-2022 period.
• The zero nitrogen treatment in rotated corn reduced yield by 37% compared to a full rate in 2007-2011, but only 4% in 2016-2022 (Figure 11).
• Visual nitrogen deficiency symptoms in the field corresponded with measured yield results.
• Distinct yellowing of the plants was observed in the lower nitrogen rates in continuous corn, with detectable deficiency symptoms in the zero nitrogen strip appearing soon after emergence.
• In the rotated corn, nitrogen deficiency symptoms were nearly nonexistent in the latter years of the study, even in the zero nitrogen strip.