Evaluating Corn Agronomics According to Nitrogen Rates and Cover Crop Mix Selection

March 26, 2025

TRIAL OBJECTIVE

Cover crops serve as an alternative method for introducing nitrogen (N) into cropping systems by facilitating atmospheric N fixation through legume species.
Managing N fertilization in cropping systems with diverse cover crop mixtures can be challenging. Some cover crops have a high carbon-to-nitrogen ratio (C:N), which may lead to N immobilization.
The objective of this trial was to evaluate corn agronomic parameters according to cover crop mixes and N management.

RESEARCH SITE DETAILS

The trial was arranged as a randomized complete block design including four replications with two treatment factors (Table 1).

Table 1: List of experimental factors, treatments, cover crop planting and termination dates, and seeding rates.

Cover crop mixes were classified according to their C:N ratio. Generally, mixtures with a higher proportion of grasses than legumes have a higher C:N ratio, leading to slower decomposition and prolonged N immobilization. Conversely, mixtures dominated by legumes have a lower C:N ratio, resulting in faster decomposition and quicker N release into the soil.¹
Field operations, corn product, herbicide, irrigation, and precipitation information are listed in Table 2.

Table 2: List of field operations, herbicide and irrigation applications, and precipitation.

Soil samples from each cover crop strip were taken when corn reached the V4 growth stage, and the results are displayed in Table 3.
Normalized Difference Vegetative Index (NDVI) was collected using a Trimble^® handheld device when corn reached the V4 growth stage.
Plant height, ear height, and final plant population were measured when corn reached the R4 growth stage.
Total weight, test weight, and moisture content were collected with a plot combine and yield per acre was later calculated.
Statistical analysis for Fisher’s Least Significant Difference (LSD) was performed.

Soil sample results in each cover crop mix strip from 0–8 and 8–24 inches soil depth.

Cover crop biomass achieved at each C:N ratio mix shortly after cover crop termination. Image from the Bayer Crop Science Learning Center at Gothenburg, NE, 2024.

Figure 1. Cover crop biomass achieved at each C:N ratio mix shortly after cover crop termination. Image from the Bayer Crop Science Learning Center at Gothenburg, NE, 2024.

UNDERSTANDING THE RESULTS

Corn Normalized Difference Vegetative Index (NDVI), and final corn stand count.

Corn normalized difference vegetative index (NDVI), final stand count, and final stand as percent of seeding rate.

High and medium C:N ratio mixes produced more cover crop biomass per acre (Figure 1) and reduced NDVI readings on corn compared to no cover crop and low C:N mix (Table 4). Lower NDVI readings indicate a lower corn biomass and/or N content on those plants.
The high C:N ratio mix had the lowest final stand count compared to all other treatments (Table 4). The high C:N ratio mix, composed by cereal rye only, achieved a final stand of 78% of the seeding rate (26,354 plants/acre from 34,000 seeds/acre planted). Lower corn final stand counts could be due to excessive cover crop biomass production making corn planting operations more difficult.

Corn Plant and Ear Height

Figure 2. Corn plant and ear height, averaged across nitrogen application rates and replications, by cover crop mix. The Bayer Crop Science Learning Center at Gothenburg, NE, 2024. Numbers followed by letters indicate statistical differences at α = 0.1.

The medium C:N ratio cover crop mix produced the highest average plant height (76.5 inches) when compared to all other treatments and produced a higher average ear height (29.4 inches) than the high C:N ratio mix (27.9 inches) and the no cover crop treatment (28.4 inches). The average ear height (29.4 inches) in the medium C:N ratio mix was similar to the low C:N ratio mix (29.0 inches) (Figure 2).

Corn Grain Yield

Figure 3. Corn grain yield, averaged across nitrogen application rates and replications, by cover crop mix. The Bayer Crop Science Learning Center at Gothenburg, NE, 2024. Numbers followed by letters indicate statistical differences at α = 0.1.

The medium C:N ratio mix had the highest average corn grain yield (253 bu/acre) compared to all other treatments (Figure 3).

Figure 4. Corn grain yield, averaged across cover crop mixture and replications, by nitrogen application rates. Gothenburg, NE, 2024.

In this study, the three N rate treatments did not produce statistically significant differences in any of the evaluated parameters (Figure 4).

Figure 5. Corn at the R5 growth stage by cover crop mix used. Image from the Bayer Crop Science Learning Center at Gothenburg, NE, 2024.

KEY LEARNINGS

A balanced C:N ratio mix can achieve a reasonable cover crop biomass, helping to decrease the risk of N immobilization while suppressing weeds and cycling nutrients.
High C:N ratio cover crop mixes can induce N immobilization. Opting for a low (or eventually a medium) C:N ratio cover crop mix may help mitigate N stress effects in corn.
Excessive growth of grass cover crops can impact corn plant stands. Incorporating legumes or cover crop mixes that include legumes is a recommended option before planting corn. To manage excessive grass biomass, consider terminating it early or using a roller crimper to flatten it.
Understanding N rate calibration for different cover crop mixtures remains essential. The Bayer Crop Science Learning Center at Gothenburg, NE will continue researching this topic.
Visit with your local Bayer agronomist for more information about cover crop and N management.

Sources

¹Ruark, M. and Franzen, D. 2020. Nitrogen availability from cover crops: Is it always about the C:N ratio? Crops & Soils. 53(1): 3–7. https://doi.org/10.1002/crso.20003

Web sources verified 03/06/25. 1213_516151