Should You Apply Pre-Tassel Nitrogen in Corn?
Rasel Parvej, Dan Fromme, Josh Copes, and Syam Dodla
Nitrogen is the most yield liming nutrient for corn production. Corn requires nitrogen for amino acids, protein, and chlorophyll production. Chlorophyll is the key component for photosynthesis. Chlorophyll deficiency results in reduced yield potential. A 200-bushel corn 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 may subject to loss to the environment through volatilization (if nor incorporated, mainly for urea), denitrification (due to water-logged anaerobic conditions), and leaching (due to excessive rainfall for coarse-textured low cation exchange capacity soils). Therefore, nitrogen management in corn is one of the biggest concerns each producer has every year. It is recommended to apply nitrogen in at least 2 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 plus other deficient nutrients (mainly phosphorus and potassium based on soil-test level) around 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 researchers showed that two applications are good enough to maximize corn yield under ideal conditions for most soils having medium to high cation exchange capacity (CEC >10), sometimes it is advised to apply nitrogen in 3 splits with 1/4 at planting, 2/4 around V5-V6 stage, and 1/4 before tasseling especially for coarse-textured soils with low CEC (<10) and for years with lots of rainfall during the early corn growing season. Including pre-tassel application in nitrogen fertilization program can help reduce nitrogen loss and ensure adequate nitrogen supply during the maximum nitrogen uptake period from V10 to tasseling. It also helps adjust nitrogen rate based on crop growth, environmental forecasts, crop sensing, and tissue testing. Many land-grant university trials showed that pre-tassel nitrogen application can increase corn yield if some pre-plant and sidedress nitrogen are lost due to excessive rainfall during early growing season (Figure 1).
Corn tissue testing is one of the important tools that guides whether pre-tassel nitrogen is required. For tissue testing, about 15-20 fully developed entire leaf below the whorl should be collected around V12 stage and sent immediately to the lab for analysis. This would allow producer enough time to get the results back and make decision. The critical (normal) corn leaf nitrogen concentration around pre-tassel stage ranges from 2.75 to 3.5%. So, leaf nitrogen concentration below 2.75% would be considered low and above 3.5% would be high. One caveat about tissue testing is, nitrogen concentration in corn leaf is highly influenced by crop growth and dilution factor; so, it may not always accurately diagnose nitrogen deficiency and indicate pre-tassel nitrogen need.
Considering excessive rainfall, crop growth, and/or tissue-testing, once producer decided to apply pre-tassel nitrogen, the application rate should not be too high at this stage especially as foliar application. Broadcasting high rate of nitrogen would burn foliage (Figure 2). The pre-tassel nitrogen rate should be 15 to 25% of the total nitrogen applied i.e. roughly 50 lb nitrogen per acre. Producer can choose dry (urea) or liquid (UAN) nitrogen source. Both dry and liquid nitrogen 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 and avoid foliage damage. A little rainfall should be expected after aerial application, which would help incorporate nitrogen fertilizer and reduce foliage burn.
Figure 1. Nitrogen deficient corn in saturated soils due to excessive rainfall. (Source: https://www.pioneer.com/us/agronomy/nitrogen_application_timing.html)
Figure 2. Corn leaf burn due to broadcasting 100 lb nitrogen per acre as UAN. Photo courtesy: John E. Sawyer, Extension Soil Fertility Specialist, Iowa State University.