Nitrogen is the most yield limiting nutrient for corn production. Corn requires nitrogen for amino acids, protein, and chlorophyll production. Chlorophyll is the key component for photosynthesis. Insufficient chlorophyll content results in reduced yield potential. A 200-bushel corn crop requires about 200 to 240 lb nitrogen per acre i.e. roughly 1 to 1.2 lb nitrogen per bushel corn harvested.
Applying all the nitrogen at or before planting are prone to loss to the environment through volatilization, denitrification, and/or leaching. Volatilization loss is high in hot and humid climates such as Louisiana and alkaline soils if the fertilizer (especially urea) is not incorporated within a few days.
Leaching loss is high in high rainfall areas especially in sandy to sandy loam soils with low cation exchange capacity (CEC). Denitrification loss is the main concern in poorly drained soils but can occur in any soils with excessive rainfall that creates water-logged anaerobic conditions. Like this year, excessive rainfall often occurs in lower Mississippi Delta during the early corn growing season resulting in saturated soils for several days and accelerates nitrogen loss via denitrification.
Therefore, nitrogen management in corn is one of the biggest concerns for producers every year. It is recommended to apply nitrogen in at least two splits during the growing season with 1/3 at planting and 2/3 around V5-V6 stage (5-6 leaves with visible collars and plant is about 12-inch tall).
Providing adequate nitrogen near V5-V6 stage is very important because corn initiates ear shoots and tassel and sets yield components at or little after V6 stage.
Although most of the research shows that two applications are good enough to maximize corn yield under ideal conditions for most soils with medium to high CEC (>10), nitrogen application in three splits with 1/4 at planting, 2/4 around V5-V6 stage, and 1/4 before tasseling could be beneficial for coarse-textured low CEC (<10) soils as well as for poorly-drained soils that are very prone to water-logged conditions.
This also helps in years with excessive rainfall during the early corn growing season, which increases nitrogen losses. Including a pre-tassel application in nitrogen fertilization program can help reduce nitrogen losses and ensure adequate nitrogen supply during the maximum nitrogen uptake period from V10 to grain-filling stage.
It also helps adjust nitrogen rate based on crop growth, yield potential, environmental forecasts, crop sensing (NDVI, SPAD, etc.), and tissue testing. Many land-grant universities including LSU AgCenter trials showed that pre-tassel nitrogen application can increase corn yield when part of the pre-plant and sidedress nitrogen are lost due to excessive rainfall during early growing season (Figure 1).
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Corn tissue testing is one of the important tools that guides whether pre-tassel nitrogen is required. For tissue testing, about 15-20 uppermost fully developed entire leaves below the whorl should be collected around V12-V13 stage and sent immediately to the lab for nitrogen concentration. This would allow enough time for the producer to get the results back and make a decision.
The critical (normal) corn leaf nitrogen concentration around pre-tassel stage ranges from 2.75 to 3.5%. Leaf nitrogen concentration below 2.75% would be considered low and above 3.5% would be high. One caveat about tissue testing is, it may not always accurately diagnose nitrogen deficiency and indicate pre-tassel nitrogen need because nitrogen concentration in corn leaf is highly influenced by crop growth and dilution factor.
For example, leaf nitrogen concentration can be high due to insufficient plant growth (low dilution) associated with drought, diseases, and pest infestation. Therefore, care should be taken interpreting leaf nitrogen concentration.
Overall, a producer should consider rainfall amount following sidedress nitrogen application, field conditions, crop growth, yield potential, and/or tissue-testing when deciding to apply pre-tassel nitrogen.
Applying high rates of urea ammonium nitrate (UAN) as a foliar application is not recommended due to the potential for severe foliage burn (Figure 2). The pre-tassel nitrogen rate should be 15 to 25% of the total nitrogen applied i.e. roughly 40 to 60 lb nitrogen per acre.
Producer can choose dry (urea) or liquid (UAN) nitrogen source. Both dry and liquid nitrogen fertilizers can be flown by airplane; but it would be better to place nitrogen close to plant base, if possible, with high clearance applicator using “360 Y-drop” to facilitate rapid uptake, minimize nitrogen losses, and avoid foliage damage.
Application before an expected rain (about 0.25-inch) or pivot irrigation is recommended to incorporate applied nitrogen that will minimize foliage burn as well as volatilization loss. Further, multiple studies conducted at LSU AgCenter showed that use of N-stabilizers improves the efficacy of applied nitrogen fertilizer up to 20%.