Field Evaluation of Sidedress Nitrogen Applications in Corn

• A field experiment was conducted in Northeast Iowa in 2024 by Heritage Ag Research to evaluate nitrogen management program effects on corn growth and yield.
• The experiment was conducted at two different seeding rates and with four different Pioneer^® brand corn products to determine if either hybrid or plant population influenced yield outcomes of different nitrogen management programs.
• Crop canopy biomass and chlorophyll levels were assessed via UAV-based remote sensing at multiple dates during the growing season to evaluate the utility of remote sensing in monitoring crop health and nitrogen status.

• Location: Field research site near Readlyn in Northeast Iowa
• Previous Crop: Soybean
• Plot Layout: Four row x 30-ft plots in a split-plot arrangement within a randomized complete block design; 5 replications.
• Seeding Rates: 32,000 and 36,000 seeds/A

Experimental Factors

• Nitrogen Treatment Program
  • 100 lbs N/A (50 lbs fb 50 lbs)
  • 150 lbs N/A (50 lbs fb 100 lbs)
  • 180 lbs N/A (50 lbs fb 130 lbs)
• Hybrid/Brand¹
  • P00549PCE™ (PW, ENL, RIB) - 100 CRM
  • P05737PCE™ (PW, ENL, RIB) - 105 CRM
  • P1027AM™ (AM, LL, RR2) - 110 CRM
  • P13050AM™ (AM, LL, RR2) - 113 CRM

• Corn was planted on May 19, which was later than normal for the location due to above-average rainfall during the spring planting window (Table 1).

  Table 1. Cumulative monthly precipitation at the research location near Readlyn, Iowa in 2024 compared to monthly averages.

  Month	2024	Average
  	----- inches -----
  April	4.0	3.9
  May	10.1	4.7
  June	7.2	4.9
  July	4.7	4.5
  August	2.0	4.1
  September	0.4	3.0

• The field study was comprised of two parallel experiments — one planted at 32,000 seeds/A and the other at 36,000 seeds/A. This design allowed comparisons of hybrid and nitrogen management programs at lower and higher seeding rates, but not direct comparisons between seeding rates.
• All nitrogen treatments were applied as sidedress injection of 32% UAN. Initial treatments of 50 lbs N/A were applied on June 14 and follow-up applications of 50, 100 or 130 lbs N/A were applied on June 26.
• Crop canopy data were collected by UAV flights conducted on August 19, September 2, September 20 and October 7.
• Canopy reflectance data from the center two rows of each plot were used to calculate three vegetation indices: normalized difference vegetation index (NDVI), leaf chlorophyll index (LCI) and modified chlorophyll absorption in reflective index (MCARI).
• The study was harvested on October 13 and the center two rows of each four-row plot used to determine yield, grain moisture and test weight.

Corn Yield

• Corn yield significantly differed among hybrids at both seeding rates (Figure 3).
• At 32,000 seeds/A, yield of Pioneer^® P00549PCE™ brand corn was significantly lower than yield of the other three corn products, an outcome likely at least partly attributable to its shorter relative maturity.
• Yield differences among hybrids were similar at the 36,000 seeds/A rate except for Pioneer P13050AM™, which yielded significantly lower than P05737PCE™ and P1027AM™ at the higher seeding rate.
• Yield was significantly affected by nitrogen management at both seeding rates, with the 100 lbs N/A program yielding less than the 150 lbs N/A and 180 lbs N/A rates (Figure 3).

  Corn Yield (bu/A)

  

  Figure 3. Corn yield by hybrid and nitrogen treatment at 32,000 and 36,000 seeds/A seeding rates. Means with the same letter within each group are not significantly different based on Student’s t test at α=0.05.

• The 180 lbs N/A program did not provide a significant yield advantage over the 150 lbs N/A program and results did not suggest the need for more nitrogen at the higher seeding rate.
• Weather conditions during the 2024 growing season may have factored into the yield results – rainfall was above average early in the season before N was applied but below average following the two application timings.
• A nitrogen experiment using the same total N rates but applied prior to planting may have produced different results, as N applied in April or May would have been at greater risk of loss through leaching and denitrification.
• The lack of a significant interaction between hybrid and nitrogen management at either seeding rate indicates that the hybrids responded similarly to nitrogen management.
• Numerous Pioneer research studies over the years have compared nitrogen rate response of different hybrids (Jeschke and DeBruin, 2016). Differences in hybrid response to N in these studies have generally been relatively minor and inconsistent, which suggests that attempting to tailor nitrogen management programs to individual hybrids is unlikely to improve yield or efficiency.

Harvest Moisture

• Grain moisture at harvest significantly differed among hybrids and was positively correlated with hybrid maturity, an outcome that was to be expected given the relatively wide range hybrid maturities in the study (Figure 4).

  Corn Grain Moisture (%)

  

  Figure 4. Corn grain moisture at harvest by hybrid and nitrogen treatment at 32,000 and 36,000 seeds/A seeding rates. Means with the same letter within each group are not significantly different based on Student’s t test at α=0.05. NS = no significant difference.

• Grain moisture of hybrids was very similar between seeding rates, except for P13050AM™, which was 2.5 points wetter at the higher seeding rate. The experimental design of this study does not allow any conclusions to be drawn as to whether this was a meaningful difference attributable to seeding rate or not.
• Nitrogen management did not significantly affect grain moisture, nor were there significant hybrid by nitrogen interactions, at either seeding rate.

Test Weight

• Grain test weight significantly differed among hybrids and was inversely correlated with hybrid maturity (Figure 5).

  Corn Grain Test Weight (lbs/bu)

  

  Figure 5. Corn grain moisture at harvest by hybrid and nitrogen treatment at 32,000 and 36,000 seeds/A seeding rates. Means with the same letter within each group are not significantly different based on Student’s t test at α=0.05. NS = no significant difference.

• Nitrogen management did not significantly affect test weight, nor were there significant hybrid by nitrogen interactions, at either seeding rate.
• Test weight was slightly lower across the board at the 36,000 seeds/A seeding rate but, again, it’s unclear given the experimental design if this was a meaningful difference attributable to seeding rate.
• Test weight in this study was likely influenced by the onset of drought stress during the grain filling period, caused by below-average rainfall in August and September.

Vegetation Indices

• There were significant differences among hybrids for all three vegetation indices at all imagery timings (Table 2).

  Table 2. Effects of hybrid and nitrogen management on vegetation indices (NDVI, LCI, and MCARI). An ‘x’ indicates that the main effect was significant at α=0.05.

  NDVI		8-19	9-2	9-20	10-7
  32K	Hybrid	x	x	x	x
  	Nitrogen	x	x
  36K	Hybrid	x	x	x	x
  	Nitrogen	x	x	x
  LCI		8-19	9-2	9-20	10-7
  32K	Hybrid	x	x	x	x
  	Nitrogen	x	x	x
  36K	Hybrid	x	x	x	x
  	Nitrogen	x	x	x	x
  MCARI		8-19	9-2	9-20	10-7
  32K	Hybrid	x	x	x	x
  	Nitrogen
  36K	Hybrid	x	x	x	x
  	Nitrogen

• Leaf chlorophyll index (LCI) was the vegetation index most affected by nitrogen management, with significant differences among nitrogen programs in every instance except in the 32,000 seeds/A seeding rate at the final imagery timing.
• NDVI significantly differed among nitrogen programs at the first two imagery timings, only in the higher seeding rate at the third timing and in neither seeding rate at the final timing.
• MCARI was not affected by nitrogen management.

• Vegetation index means by hybrid and nitrogen treatment from the August 19 imagery timing are shown in Figure 7, Figure 8 and Figure 9.
• All three vegetation indices showed significant differences among hybrids. NDVI had the greatest degree of statistical separation, with all hybrids differing significantly from each other at both seeding rates (Figure 7).

  Normalized Difference Vegetation Index (NDVI)

  

  Figure 7. Normalized difference vegetation index (NDVI) by hybrid and nitrogen treatment at 32,000 and 36,000 seeds/A seeding rates on August 19, 2024. Means with the same letter within each group are not significantly different based on Student’s t test at α=0.05.

• LCI showed less statistical separation among hybrids compared to NDVI, but generally similar patterns (Figure 8).

  Leaf Chlorophyll Index (LCI)

  

  Figure 8. Leaf chlorophyll index (LCI) by hybrid and nitrogen treatment at 32,000 and 36,000 seeds/A seeding rates on August 19, 2024. Means with the same letter within each group are not significantly different based on Student’s t test at α=0.05.

• MCARI differed from the other two indices — at 32,000 seeds/A, P1027AM™ was significantly greater than the other three hybrids, while at 36,000 seeds/A P13050AM™ dropped off compared to the other hybrids (Figure 9).

  Modified Chlorophyll Absorption in Reflectance Index (MCARI)

  

  Figure 9. Modified chlorophyll absorption in reflectance index by hybrid and nitrogen treatment at 32,000 and 36,000 seeds/A seeding rates on August 19, 2024. Means with the same letter within each group are not significantly different based on Student’s t test at α=0.05. NS = no significant difference.

• When comparing vegetation indices to yield, hybrid means often ranked in similar order, but significant differences in vegetation indices often occurred where there were no corresponding differences in yield.
• For nitrogen treatments, both NDVI and LCI corresponded with yield results, with values for the 100 lbs N/A treatment significantly lower than the other two rates.
• MCARI did not differ among nitrogen treatments at either seeding rate.
• Figure 10, Figure 11 and Figure 12 show how vegetation indices changed at later imagery timings. All three declined toward the end of the season, with the greatest decline in late-September to early October as the canopy senesced.
• NDVI remained relatively unchanged until the final imagery timing on October 7 (Figure 10), while both LCI and MCARI had noticeable declines by September 20 (Figure 11 and Figure 12).

  Normalized Difference Vegetation Index (NDVI)

  

  Figure 10. Normalized difference vegetation index (NDVI) by hybrid at 36,000 seeds/A seeding rate.

  Leaf Chlorophyll Index (LCI)

  

  Figure 11. Leaf chlorophyll index (LCI) by hybrid at 36,000 seeds/A seeding rate.

  Modified Chlorophyll Absorption in Reflectance Index (MCARI)

  

  Figure 12. Modified chlorophyll absorption in reflectance index (MCARI) by hybrid at 36,000 seeds/A seeding rate.

• The primary interest in vegetation indices in this study was in their utility for assessing nitrogen status and predicting associated yield outcomes.
• Results suggest that NDVI and LCI could be useful for this purpose, but differences in these indices among hybrids were equal to or greater than those among nitrogen treatments, suggesting the need to calibrate predictions to individual hybrids.

Jeschke, M., and J. DeBruin. 2016. Corn Hybrid Response to Nitrogen Fertilizer Pioneer Crop Insights. Vol 26, No. 8. Corteva Agriscience. Johnston, IA.