Rasel Parvej, Syam Dodla, and Matthew Foster

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 250 pounds nitrogen per acre i.e., roughly 1 to 1.25 pounds nitrogen per bushel corn harvested. The upper range is for clayey soils since corn production in clayey soils requires more nitrogen than sandy or silty soils due to nitrogen (ammonium ion) fixation between clay particles to a biologically unavailable form.

Nitrogen management in corn production is one of the biggest concerns for corn producers every year. Nitrogen is recommended to apply in 2 to 3 splits from planting to tasseling since it is very prone to loss to the environment via different loss mechanisms. Unfortunately, most corn producers in Louisiana apply the total nitrogen fertilizer in a single application as sidedress at or few weeks after corn emergence. A significant amount of this nitrogen in most years can be lost during the growing season through volatilization, denitrification, leaching, and/or runoff, resulting in corn yield loss. Volatilization loss is very high in hot and humid climates, common in Louisiana, and in alkaline soils (pH more than 7.0) if nitrogen fertilizer (especially urea but can be UAN as it contains 50% urea) is not incorporated within a few days after application. Denitrification loss is the main concern in poorly drained soils but can occur in any soil with excessive rainfall that creates water-logged anaerobic conditions. Leaching loss is high in high rainfall areas especially in sandy soils with low cation exchange capacity (CEC). In most years in Louisiana, excessive rainfall often occurs in the lower Mississippi Delta during the early growing season, resulting in saturated soils for several days, which accelerates nitrogen losses via denitrification, leaching, and/or runoff and reduces corn yield potential. Although researchers from the mid-South states have showed that it is possible to maximize corn yield by a single nitrogen application during the growing season in both silt loam and clay soils, for this to occur, the growing season must be ideal with moderate temperature and adequate and evenly distributed rainfall, which is seldom occurred in Louisiana. Since we cannot predict weather conditions during the growing season, a single application is, therefore, a risky nitrogen management plan for corn production in most years in Louisiana.

In general, a 200-bushel corn in silt loam soils requires about 10% (20 pounds) of the total nitrogen (200 pounds) from planting to V6 stage (6 visible collar leaves and plant is about 12-18 inches tall; Figure 1), 55% (110 pounds) from V6 to R1 (silking), and 35% (70 pounds) from R1 to R6 (maturity; Figure 2). On the other hand, corn initiates ear shoots and tassel and sets yield components such as kernel rows per ear and potential kernels per row at or little after V6 stage (Figure 2). Considering both nitrogen requirement and yield component development at different growth stages of corn, it seems like a nitrogen management plan should include nitrogen application in three splits with a small amount of nitrogen (1/4) at planting, most (2/4) at around V6 stage, and another small amount (1/4) at pre-tassel stage. However, most of the research showed that two applications (1/4 at planting and 3/4 at V6-8 stage) are good enough to maximize corn yield under normal conditions in most soils with medium to high CEC (>10).

Regardless of split application number, applying a small amount of nitrogen at or before planting would provide the corn plant enough nitrogen for setting maximum yield components at or after V6 stage. It would also provide a wide window of opportunity to sidedress nitrogen application during the growing season from V6 to V8 stage. For instance, having a pre- or at-planting nitrogen application would allow the producers to delay their sidedress application if missed at V6 stage due to rainfall and wet soil conditions. Unfortunately, pre- or at-planting nitrogen application is not very common in Louisiana corn production. Rather, most corn producers in Louisiana often use in-furrow starter fertilizer (ammonium polyphosphate 10-34-0 or 11-37-0 at a rate of 5 gallons per acre) to compensate corn nitrogen requirements during the early growing season. However, this starter fertilizer is not adequate to meet the early season nitrogen demand of about 30 pounds nitrogen per acre because a 5-gallon starter fertilizer (10-34-0) per acre provides 19.8 pounds phosphorus (P2O5) but only 5.8 pounds nitrogen. Corn nitrogen demand during the early season can only be met by applying nitrogen as broadcast (urea; 46-0-0) followed by incorporation before planting or as sidedress or dribbling (UAN; 32-0-0) at planting. Note that pre- or at-planting nitrogen rate should not exceed 45 pounds per acre for silt loam and 60 pounds per acre for clay soils and should not be applied in the seed furrow due to the likelihood of potential salt injury from ammonium-nitrogen.

Sometimes a 3rd application of nitrogen (around 45-50 pounds per acre) at pre-tassel stage (V12-V14; about 2 weeks prior to tassel) is beneficial especially 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 protect corn yield losses in years with excessive rainfall during the early corn growing season, which increases nitrogen losses. Including a pre-tassel application in the nitrogen management program can help reduce nitrogen losses and ensure adequate nitrogen supply during the maximum nitrogen uptake period from V10 to grain-filling stage. 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 the early growing season. However, the need for a 3rd nitrogen application at or before tasseling should be based on crop growth, rainfall amount and soil conditions during the growing season, yield potential, environmental forecasts, reference strips (NDVI), and leaf tissue testing (leaf nitrogen concentration). To determine the need for a pre-tassel (V12 to 14) nitrogen application, a nitrogen reference strip can easily be established by applying a double rate of the total nitrogen in one corner of the corn field and/or leaf tissue sample can be collected from V10 to tasseling stage and analyzed for total nitrogen concentration. A detailed article regarding corn pre-tassel nitrogen application will be posted in April or May newsletter issues. Overall, an ideal nitrogen management program for over 200-bushel corn yield should include at least 30 to 45 pounds nitrogen at planting and remainder amount at V6 to V8 stage with or without 45 to 50 pounds nitrogen before tasseling based on NDVI reading from reference strips and/or leaf nitrogen concentration.