Nitrogen (N) is the most yield limiting nutrient for corn production. The LSU AgCenter recommends 1 to 1.25 lbs of N per bushel of corn harvested i.e., a 200-bushel corn crop requires about 200 to 250 lbs N/acre. Corn in clayey soils requires more N for the same yield goal than corn in sandy to silty loam soils.
This is mainly because clay fixes more fertilizer-N (ammonium ion, NH4+) to a biologically unavailable form and has complex N uptake route from fertilizer-N to plant roots due to the presence of a huge amount of micropores compared to sandy or silty loam soils.
The upper range of recommendations i.e., 1.25 lb N/bu. corn yield is, therefore, for clayey soils and the lower range i.e., 1 lb N/bu. corn yield is for sandy to silty loam soils.
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 in the environment via different loss mechanisms. Unfortunately, many corn producers in Louisiana apply the total N fertilizer in a single application as sidedress at or few weeks after corn emergence.
A significant amount of this early N 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 N 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 early in the growing season, resulting in saturated soils for several days, which accelerates N 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 N 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 seldom occurs in Louisiana.
Since we cannot predict weather conditions during the growing season, a single application is, therefore, a risky N management plan for corn production in most years in Louisiana.
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In general, a 200-bushel corn requires about 15% (~35 lbs) of the total N from planting to V6 stage (6 visible collar leaves and plant is about 12-18 inches tall), 60% (~150 lbs) from V6 to R1 (silking), and 25% (~65 lbs) from R1 to R6 (maturity; Figure 1). Corn sets yield components such as kernel rows/ear from V1 to V7 stages, potential kernels/row from V7 to tasseling, and harvestable kernels/row from tasseling to R3 stages (Figure 2).
Considering both N requirement and yield component development at different growth stages of corn, N should be applied in 3-splits with a small amount of N at planting, most at around V6 stage, and another small amount at pre-tassel stage to achieve maximum N use efficiency and corn grain yield.
Applying a small amount of N (30 to 45 lbs) at or before planting would provide the corn plant enough N for setting the most important yield components of kernel rows/ear from V1 to V7 stages and potential kernels/row from V7 to V15 stages (Figure 2).
It would also provide a wide window of opportunity to the sidedress N application from V6 to V8 stages. For instance, having a pre- or at-planting N 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 N 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 @ 5 gal/acre) that provides 19.8 lbs phosphorus (P2O5) and only 5.8 lbs N and would not be able to compensate early season corn N requirements.
Corn early season N demand (30 to 45 lbs/acre) can only be met by either applying 65 to 98 lbs of urea (46-0-0) or 8.5 to 12.7 gals of UAN (32-0-0) per acre at planting. If 10-34-0 or 11-37-0 starter fertilizer is used, producers need to apply 26 to 39 gals/acre to fulfill the 30 to 45 lbs N need at planting, which would not be economically sound since fertilizer price is at a record high.
Although most of the research showed that N application in 2-splits (planting and V6-8) is good enough to maximize corn yield under normal conditions in most soils with medium to high CEC (>10), a 3-way split of N with a 3rd application (~45 to 60 lbs N/acre) at or before tasseling stage (V11-V13; about 2 weeks prior to tassel) is recommended especially for coarse-textured low CEC (<10) soils as well as for poorly drained soils that are very prone to water-logged conditions.
This helps protect corn yield losses in years with excessive rainfall during the early corn growing season, which increases N losses. Researchers from many land-grant universities including LSU AgCenter found that pre-tassel N application can increase corn yield when part of the pre-plant and sidedress N are lost due to excessive rainfall during the early growing season.
However, for fields that already received all the N fertilizer in 2-splits, the need for a 3rd N application at or before tasseling should be based on soil type, crop growth, rainfall amount and soil conditions during the growing season, yield potential, environmental forecasts, reference strips (NDVI), and/or leaf N concentration.
The NDVI reading from N reference strips and leaf N concentration from V10 to tasseling stage can be used to determine the need for a pre-tassel (V12 to 14) N application. A detailed article regarding corn pre-tassel nitrogen application will be posted in May newsletter issues.
Overall, an ideal N management program for over 200-bushel corn yield/acre should include at least 30 to 45 lbs N at planting and the remaining amount at V6 to V8 stage with or without 45 to 60 lbs N before tasseling based on NDVI reading from reference strips and/or leaf N concentration.