Tissue testing helps determine in-season nitrogen loss and pre-tassel nitrogen requirements in corn

Rasel Parvej

Assistant Professor and State Soil Fertility Specialist, LSU AgCenter

Core Ideas:

• Leaf tissue testing is one of the best indicators of determining N losses during the growing season.
• Leaf tissue testing before tasseling indicates the pre-tassel N requirement for maximizing corn yield.
• Leaf tissue sampling should be done from V10 (10 collar leaf stage) to R1 (silking) stage.
• The sufficient leaf-N concentration from V10 to R1 (silking) stage should be over 3.0%.

In the lower Mississippi Delta, excessive rainfall is common during the early growing season, leading to saturated soils for several days. This condition accelerates nitrogen (N) losses through denitrification, leaching, and runoff, thereby reducing corn yield potential. Consequently, the LSU AgCenter recommends applying N in at least two splits for silt loam and clayey soils, and in three splits for sandy soils. This approach involves a small application at planting (either 2x2 or dribbled on top of the bed), followed by the majority at the V5-6 stage (when the plant has 5-6 leaves with visible collars and is about 12 to 18 inches tall) as a sidedress for silt loam and clayey soils, and an additional small amount before tasseling, especially for sandy soils. For instance, for a 200-bushel corn crop on silt loam soils, 200 lbs of N per acre (1 lb N per bushel of corn yield per acre for sandy to silt loam soils and 1.25 lbs N for clayey soils) should be split into approximately 30 to 40 lbs at planting and 160 to 170 lbs at the V5-6 stage. While research from the mid-South states indicates that maximizing corn yield with a single N application during the growing season is possible in both silt loam and clay soils, such a strategy requires ideal growing conditions with moderate temperatures and evenly distributed rainfall, which rarely occur in Louisiana. Therefore, relying on a single N application is typically a risky management plan for corn production in most years in Louisiana.

At any given stage, it is challenging to measure how much N has been lost from corn fields because N losses due to excessive rainfall depend on various factors such as soil type, drainage, and cation exchange capacity (CEC). Although N can be lost through different mechanisms, denitrification is the primary concern during periods of excessive rainfall, especially in poorly drained soils. However, denitrification can occur in any soil that becomes waterlogged and anaerobic due to excessive rainfall. In well-drained corn fields without waterlogged conditions, denitrification is less of a concern, though N leaching can still be an issue with high rainfall. Leaching is particularly common in sandy soils with low CEC (<10). Fortunately, N leaching primarily affects nitrate-N (NO₃-N) fertilizer. Since urea ammonium nitrate (UAN; 32-0-0, 30-0-0-2S, or 28-0-0-5S) is the most commonly used N fertilizer for corn production in Louisiana, and it contains only 25% nitrate-N, the maximum potential leaching loss from UAN is limited to 25% of the total N applied.

Corn tissue testing is a crucial tool for assessing the N status of corn during the growing season and determining if a pre-tassel N application is needed to maximize yield. To evaluate N losses due to excessive rainfall after sidedressing, producers should wait until the V10 stage (10-collar leaf stage) to take tissue samples. Sampling can be done from the V10 to the R1 (silking) stage, but it is preferable to sample earlier (at V10) if the fields have experienced several days of waterlogging. For tissue testing, collect the uppermost fully developed leaf with a visible collar below the whorl (Figure 1) from 10-15 plants and send the samples immediately to the lab for total N concentration analysis. In large fields, collect several composite tissue samples from different areas to better understand the overall N status and guide subsequent management decisions.

The critical leaf nitrogen (N) concentration for corn from the V10 to R1 stage is 3.0% (Figure 2). A leaf N concentration below 3.0% is considered deficient, indicating that additional N is needed to maximize yield. Conversely, a concentration above 3.0% is sufficient, meaning no additional N is required. When collecting and interpreting leaf tissue samples, care must be taken because high leaf N concentrations can result from insufficient plant growth (low dilution) due to drought, diseases, or pest infestations. If the leaf N concentration is near the critical level, a pre-tassel N rate of 15 to 25% of the total N applied, or approximately 40 to 60 lb N/acre (87 to 130 lb urea/acre), is recommended. However, if the tissue N concentration is significantly low, around 2.5% or lower, a higher pre-tassel N rate is needed. For tissue concentrations between 2.0% and 2.5%, at least 100 lb N/acre should be applied.

Producers have the option to use either dry (urea, 46-0-0) or liquid (UAN, 32-0-0, 30-0-0-2S, or 28-0-0-5S) nitrogen (N) sources. Urea can be easily applied via airplane, while UAN should be applied as a surface treatment because high rates of undiluted UAN as a foliar application can cause severe foliage burn. Alternatively, UAN can be applied through a pivot irrigation system, if available, as fertigation. Regardless of the N source, it is preferable to place the N fertilizer close to the plant base, if possible, using a high-clearance applicator with “Y-drops” to facilitate rapid uptake, minimize N losses, and avoid foliage damage. Applying N before an anticipated rain (0.25-0.5 inch) or pivot irrigation is recommended to incorporate the N and minimize volatilization loss. Additionally, using N stabilizers (urease inhibitors) can further reduce volatilization loss. Experiments conducted by the LSU AgCenter showed that when urea was surface applied at late growth stages, the use of N stabilizers reduced ammonia volatilization losses by 74% and increased corn grain yield by 12 to 25% compared to uncoated urea. Overall, producers should consider rainfall amounts following sidedress N application, field conditions, crop growth, yield potential, and tissue testing to evaluate N losses and determine pre-tassel N application. The effectiveness of pre-tassel N application depends on accurate tissue sampling at the correct growth stage, proper interpretation of leaf N concentration, and applying the right N rate at the optimal time and method.

Figure 1. Corn leaf collar. Photos were taken from (A: sites.udel.edu/agronomy; B: webapp.agron.ksu.edu and C: edis.ifas.ufl.edu).

Figure 2. Critical leaf nitrogen concentration from V10 to R1 stages of corn. (Source: Dr. Trent Roberts, University of Arkansas)

Corn Disease Update

Trey Price and Boyd Padgett, LSU AgCenter pathologists

Article Highlights:

• Corn rust identification
• Management considerations

Common rust has been reported in some fields in central Louisiana and on the Dean Lee Research and Extension Center. However, this disease has not been a major concern in previous years.

Common rust

Common rust may be the first disease found in corn fields and usually occurs in the lower-to-mid-canopy. Disease development is favored when temperatures are cool (60-77^oF) and leaf wetness of 4-6 hours. Pustules of common rust are brick red to dark orange, somewhat elongated, and will appear on both leaf surfaces (Figures 1 and 2).Common rust will progress during relatively cool, rainy, and cloudy weather; however, very rarely are fungicide applications warranted for common rust. Warmer temperatures will greatly slow common rust development.

Southern rust

Southern rust pustules are more orange than brick red, usually not as elongated, and usually appear on the upper surface of leaves (Figure 3 and 4). The disease usually appears in mid to late season and is favored by temperatures in the upper 70’s to 90^oF and leaf wetness of 6 hours. Like common rust, the disease usually initiates in the lower-to-mid-canopy. The disease can reach the upper-canopy during conditions favorable for development. Fungicides may be justified but should be made on a field-by-field basis. The genetic resistance of the hybrid, growth stage (post tassel), current environmental conditions, and potential economic benefits are factors to consider prior to applying a fungicide.

Fungicide considerations

Fungicide application decisions should be carefully considered field by field based on: disease severity (Figure 6), crop stage (Table 1), hybrid susceptibility, fungicide efficacy, tillage regime, prevailing environmental conditions, previous experience, commodity price, and the probability of a return on the investment. If applications are warranted, apply at labeled rates using maximum (5 GPA by air, minimum) water volume is recommended.

Table 1.Percent yield loss because of defoliation by crop stage. For example…30% defoliation at dent stage results in a 2% yield loss. ^aDefoliation

Figure 1.Common rust.

Figure 2.Common rust.

Figure 3.Southern rust.

Figure 4.Southern rust.

Rootless corn syndrome (RCS) reported in many Louisiana corn fields

Trey Price and Boyd Padgett, LSU AgCenter Scientists

Soon after planting most producers received excessive amounts of rainfall over an extended period. We have received numerous field calls involving corn plants with poor nodal root development causing them to fall over (Figures 1 & 2).The mesocotyl (first true stem) can be stressed or broken in the process. The last time this phenomenon significantly occurred was 2016 (LINK).Producers that planted the higher end of plant populations will likely incur tolerable losses due to RCS. Shallow planting depth can contribute to RCS.

Plants can recover from RCS as long as they survive the lodging event and subsequent conditions (adequate rainfall) allow for nodal root development. Moving moist soil around exposed roots may help with recovery.

Figure 1 (plant death as a result of RCS); Figure 2 (normal vs. poor nodal root development).

Interestingly, damping off (Rhizoctonia solani) has been commonly observed in RCS situations where fields had been planted for at least one month (V3-V4).Over time, seed treatment efficacy declined, plants were stressed (particularly at the mesocotyl), and the pathogen took advantage of optimal environmental conditions. Classic damping off lesions have been observed on the upper sections of mesocotyl (Figures 3 & 4), and the pathogen has been successfully isolated in the laboratory.

Figure 3 (damping off); Figure 4 (lesion on mesocotyl).

Soybean Mid-May Planting Progress and Managing Flooded Conditions

David Moseley and Boyd Padgett, LSU AgCenter Scientists

Article Highlights:

• Heavy rains stall soybean planting
• Target a soybean stand of 70,000 to 75,000 when considering if additional plants are required
• A few management tips for flooded fields

Planting Progress and Delays

Louisiana's soybean planting progressed well in mid-April, exceeding the five-year average. However, heavy rains in mid-May slowed progress, with only 69% planted by May 12th, 2024. This lags behind last year's 76% at the same point. An article, “The Farmer's Forecast: More Soybean Planting Delays” indicates continued rain and potential wind/hail threats.

Emergence and Replanting Considerations

Excessive rain has also hampered emergence. LSU AgCenter research suggests a final soybean stand of at least 70,000-75,000 plants per acre is adequate for achieving 95% yield potential.

Flood Impact on Soybeans

Flooding and saturated soil conditions can significantly damage soybeans. The severity depends on plant growth stage, flood duration, and other environmental factors.

• Soybean Sensitivity: Plants become more vulnerable to flooding as they reach the R3 (pod development growth stage).
• Flood Tolerance: Depending on temperature, soybeans may withstand flooding for 2-3 days.
• Oxygen Depletion: Flooded soils lose oxygen, especially in hot weather, harming plant and microbial respiration.
• Photosynthesis Reduction: Debris covering leaves after a flood can reduce yield by limiting photosynthesis.

Managing Flooded Fields

• Drainage: If possible, clear obstructions in ditches or furrows to promote drainage.
• Traffic Control: Limit unnecessary traffic on saturated fields to prevent soil compaction.
• Foliar Applications: Reduce applications that can burn leaves after flooding.
• Scouting and Monitoring: Continue scouting for pests and assessing nitrogen-fixing nodule activity. Floods can temporarily reduce nodule activity, but it may recover afterward.

Soybean Flood Tolerance Varieties

The LSU AgCenter collaborates with universities to develop flood-tolerant soybean varieties. Data from 2023 flood trials is available in the 2024 Soybean Variety Yields & Production Practices. Similar trials are planned for 2024.

Figure 1. Soybean plants in flooded conditions due to excessive precipitation.

LSU AgCenter Specialists