Corn Biomass Allocation and Nitrogen Uptake: Insights from 13 Years of Field Research

As corn yield potential continues to increase, it is important that management practices keep pace with the changes, ensuring that yield potential is fully realized. Crops producing more grain also produce more non-harvestable biomass, both of which require more nutrient uptake from the soil. Nitrogen (N) is one of the most important inputs for corn production, as well as one of the most challenging to manage efficiently.

Plant biomass and nitrogen content were measured from a series of Corteva Agriscience field experiments conducted over a period of 13 years from 2011 to 2023. The purpose of this research was to better understand corn biomass partitioning and N utilization at different yield levels. Yield outcomes in a given environment are shaped by numerous factors, including soil characteristics, growing season conditions, and agronomic management. The purpose of this study was not to test the impact of any specific environmental or management factor on corn yield, but rather to understand how corn plants function at different yield levels in terms of taking up and allocating nitrogen and building grain yield.

Plant biomass, N content, and corn yield measurements were collected from 6,092 research plots grown in fully irrigated and rainfed locations in the Midwestern U.S., Woodland, CA, and Viluco, Chile between 2011 and 2023 (Figure 1). Soil samples were taken prior to planting at each research location to measure soil texture, organic matter, and nitrate content. Whole plant samples were collected at flowering (R1) and maturity (R6) and separated into individual plant parts to determine biomass and N content. Plant samples were processed at Corteva research facilities and nitrogen analysis was conducted in Johnston, IA.

Research plots represented a range of different growing environments and management practices. Table 1 lists research locations by year, with previous crop, irrigation status, and soil organic matter level at each site. Organic matter ranged between 1.8 and 6.7% with an average of 3.8%. Trials conducted in Woodland, CA and Viluco, Chile were fully irrigated while trials in the Midwestern U.S. were mostly rainfed, with irrigation at the San Jose, IL, Mazon, IL, Johnston, IA, Union City, TN, and York, NE locations.

Table 1. Year, location, previous crop, irrigation status, and soil organic matter for field trials conducted between 2011 and 2018 to evaluate corn yield and associated yield components, plant biomass, and nitrogen uptake.

Table 1a. Year, location, previous crop, irrigation status, and soil organic matter for field trials conducted between 2018 and 2023 to evaluate corn yield and associated yield components, plant biomass, and nitrogen uptake.

Plant densities of research plots ranged between 26,000 and 50,000 plants per acre and nitrogen fertilizer application rates up to 600 lbs N/acre (Table 2). Several experiments in the Midwest included a zero N treatment to determine the potential of the soil to contribute N via mineralization.

Table 2. Minimum, maximum, and average N rate and plant density for experimental plots that were grown between 2011 and 2023 in the Midwestern U.S., Woodland, CA, and Viluco, Chile to measure yield, plant biomass, and N uptake.

Plant biomass and N traits were categorized based on the associated yield for each test plot and split into four yield classes: <150, 150-200, 200-250, and >250 bu/acre. Measurements derived from plant samples included:

• Total N removed from field at harvest
• Total N remaining in the residue at harvest
• N uptake before and after flowering
• Amount of stored N remobilized to support grain filling
• Yield components (ovules, silks, kernels, and kernel weight)

Leaf Area Index

Leaf area index (LAI) is a measure of the total surface area of leaves per unit of ground area. Leaf area index increased with each successively higher yield class, from an average of 4.4 in the <150 bu/acre class up to an average of 6.1 in >250 bu/acre yield class. Observed leaf area index in the study ranged from 1.85 to 9.08. Greater leaf area is an indicator of improved growth and canopy expansion and increases the potential for light interception. Additionally, leaf area is a storage warehouse for nitrogen which can contribute to remobilization for grain filling.

Yield Components

The total number of ovules produced on the primary ear sets the yield potential of the plant. Ovule number is a product of the number of kernel rows around the ear, which is set at approximately the V7 growth stage, and the length of the ear (kernels per row) which is set during late vegetative growth. Ovule number in this study was relatively consistent across the higher yield classes (150-200, 200-250, >250 bu/acre), at around 680 ovules per ear. Only the lowest yield class (<150 bu/acre) had a lower ovule number, with around a 7% reduction compared to the higher yield classes.

Ideally, each ovule would produce a silk, but that did not prove to be the case. The number of silks expressed was around 20% less than the total number of ovules, an outcome that was relatively consistent across all yield classes. Leading into pollination, yield classes above 150 bu/acre had no loss in potential yield; only the lowest yield class already had lower yield potential locked in due to lower ovule number and silk expression.

Kernel number is set by the R2 growth stage, which occurs approximately 2 weeks after flowering, and can be negatively impacted by stress to the plant during this period. At yield levels above 200 bu/acre, kernel number was maximized, with essentially all expressed silks resulting in successful kernel set. Below 200 bu/acre, incomplete kernel set resulted in lost yield potential. At 150-200 bu/acre the number of kernels set was around 10% less than the number of silks expressed. Below 150 bu/acre, this gap grew to 20%.

Based on kernel number, yield classes above 200 bu/acre had no loss in yield potential at the R2 stage, while yield potential was reduced by 11% in the 150-200 bu/acre class, and by 28% in the <150 bu/acre class. Kernel weight was the yield component with the greatest degree of differentiation among yield classes, with kernel weight increasing with each successively higher yield class. Kernel weight increased 18%, 12% and 11% from the lowest yield class to the >250 bu/acre class. Kernel weight was the main component that separated good yield (200-250) from great yield (>250).

Table 3. Leaf area index of the canopy at R1, yield components at R1 (ovule and silk number), yield components at R6 (kernel number and kernel mass), and grain yield for the four yield classes. The columns following the yield trait class indicate the number of observations that were used to calculate the mean.

Because kernel weight increased with yield, the number of kernels required to reach one bushel (i.e. 56 lbs) decreased. When estimating yield from kernel number per ear and plant density select a number of kernels per bushel that is appropriate for the likely yield level.

Nitrogen Uptake

Total nitrogen uptake, measured at both the R1 and R6 growth stages, increased with each yield class (Table 4). N uptake post-flowering ranged from 43 lbs N/acre for the <150 bu/acre yield class to over 100 lbs N/acre in the >250 bu/acre yield class. The amount of N remobilized to grain increased with each yield class; however, the percentage of total grain nitrogen that came from remobilization decreased with higher yield classes, indicating that higher-yielding crops rely more on nitrogen uptake during the grain-filling period rather than remobilization.

Table 4. Nitrogen uptake by a corn crop at R1 and R6, the amount of N contained in the grain at R6, the amount of N taken up post-flowering, the amount of N remobilized from the vegetative tissue to support grain N on a lbs/acre and percent basis for four yield classes based on bu/acre. The columns following the yield trait class indicate the number of observations that were used to calculate the mean.

Measurements of total plant N and grain N at R6 indicate that a corn crop taken for silage would remove between 106 and 302 lbs N/acre depending on the yield level. Corn taken for grain would remove between 66 and 209 lbs N per acre depending on the yield level. Residue remaining in the field following grain harvest ranged between 40 and 93 lbs N per acre depending on the yield level.

Both leaves and stems are major sinks for nitrogen prior to R1. After R1 leaves and stems begin to remobilize stored N to the grain to help with grain filling. At low yield levels, the leaves contribute the majority (>50%) of N contained in the grain, with the stem contributing another 10%. At the highest yield levels, the leaves contribute a smaller portion of the grain N via remobilization (<30%).

Biomass Partitioning (R1)

At flowering (R1), stalk tissue accounted for around 60% of total aboveground biomass, with approximately 30% of the biomass in green leaves, a ratio that was stable across yield levels. Stalk biomass and green leaf biomass both increased with each successively greater yield class. The amount of biomass comprised of senesced leaves decreased with higher yield levels, even as total leaf biomass increased, indicating that plants in higher yield classes retained more green leaf tissue post-flowering.

Despite accounting for only 30% of total above-ground biomass at R1, green leaf tissue contained around 50% of total N due to a greater concentration of N in the leaf tissue compared to stalk tissue. Leaf N concentration at R1 was 2.8% for high yield classes and 2.0% for low yield classes, providing a potential sufficiency level when tissue sampling.

Biomass Partitioning (R6)

Total plant biomass increased dramatically for all yield classes between R1 and R6, with the grain accounting for around half of total above-ground biomass at maturity (Table 6). Total plant biomass (vegetative + grain) for the highest yield class was over 31,000 lbs/acre and contained over 330 lbs N/acre, both of which were more than double the averages of the lowest yield class. At maturity, over 60% of the total plant N was in the grain, with 11-15% in the stalk and another 11-16% in green and senesced leaves.

Average N concentration in the vegetative material was 0.38, 0.54, 0.65, and 0.81% for the <150, 150-200, 200-250, and >250, respectively. When the grain and cob are removed, for earlage, the N concentration in the material is 0.9, 1.0, 1.1, and 1.2% for the low to high yield categories, respectively.

Table 5. Plant biomass and nitrogen content for corn at flowering (R1) for four yield classes. The percent N concentration for each plant part and the plant part contribution to total plant biomass and N content are included. Count represents the number of observations included in the mean.

Table 6. Plant biomass and nitrogen content for corn at maturity (R6) for four yield classes. The percent N concentration for each plant part and the plant part contribution to total plant biomass and N content are included. Count represents the number of observations included in the mean.

Nitrogen Uptake

Prior to R1, N uptake (i.e. soil N contribution) ranged between 75 and 119 lbs of N per acre (Table 7). In the <150 bu/acre category, N uptake post flowering was 24 lbs/acre and increased to 38 lbs per acre in the 150-200 bu/acre category. While the number of observations is lower in the 200-250 bu/acre and >250 bu/acre categories N uptake was 68 and 122 lbs/acre, respectively.

Table 7. Nitrogen uptake in corn biomass at R1 and R6 when zero N was applied, and all N was supplied by residual NO₃-N and mineralization, for four yield classes. Count represents the number of observations that were used to calculate the mean. All measurements occurred at Midwestern U.S. locations and does not include Woodland, CA or Viluco, Chile.

This finding would indicate that field or locations within fields that can continue to mineralize N throughout the season to deliver N for grain filling will support increased grain yield. In the <150 bu/acre category, 76% of nitrogen contribution from the soil occured by R1 with little N uptake up post flowering. In the higher yield potential classes 56% nitrogen contribution from the soil occured by R1 with significant N accumulation still occurring after flowering.

Harvest Index

The harvest index (HI) is a measure of how efficiently a plant converts its total biomass into grain yield. Historically, the harvest index for corn has been around 0.5, meaning that around half of the total above-ground biomass at maturity is contained within the grain. Some studies have shown an increase in harvest index associated with genetic yield gain in corn. Results of this study showed that harvest index increased with yield level, from 0.45 at yields below 150 bu/acre, to 0.53 at 150-200 bu/acre yields, and leveling off at 0.56 for yields above 200 bu/acre (Table 8).

The nitrogen harvest index (NHI) measures how efficiently a plant converts the total nitrogen uptake into grain nitrogen. Nitrogen Harvest Index was lower for the lowest yield class in the study, averaging 0.61 and was around 0.70 for all other yield classes.

Table 8. Harvest index (ratio of grain and total biomass) and nitrogen harvest index (ratio of N contained in the grain and total N in all plant biomass) at R6 for four yield classes. Count indicated the number of observations that were used in calculating the mean.

• Production of plant biomass and the accumulation of N are strongly correlated to grain yield.
• Management that removes growth limitations prior to R1 is critical in the attempt to maximize kernel number and limit yield reductions.
• Late season N uptake from the soil rather than a dependency on remobilization is a noticeable feature of high-yield locations, contributing to kernel weight increase.

The authors would like to acknowledge Ross Allen, Angela Bryant, John Everard, Robin Roembke, and Joe Schupp for their dedicated effort over many years to provide N content data for all the plant tissues.

• ¹Rebecca Hensley, M.S., Sr. Research Associate; Corteva Agriscience, Windfall, Indiana, USA
• ²Andrea Salinas, Research Scientist; Corteva Agriscience, Viluco, Chile
• ³Andres Reyes, Ph.D., Research Scientist; Corteva Agriscience, Woodland, California, USA
• ⁴Christian Michel Navarro, Sr. Research Associate; Corteva Agriscience, Woodland, California, USA
• ⁵Logan Anderson, Sr. Research Associate; Corteva Agriscience, Johnston, Iowa, USA
• ⁶Jason DeBruin, Ph.D., Global Biotech Trait Characterization Project Scientist; Corteva Agriscience, Johnston, Iowa, USA