Sidedress and Pre-Tassel Nitrogen Application in Corn

  • Sidedressing is used to provide corn (Zea mays) with nitrogen (N) closer to when plant need and its utilization is at peak. Peak utilization occurs near the reproductive stages of crop.
  • Growers should be aware of sidedress management options and injury concerns as they sidedress N during the spring season.
  • Depending on environmental conditions and climate, different N sidedressing applications may be used to improve N availability to corn during peak growth stages.
  • Methods for N application during the growing season include injection of anhydrous ammonia, broadcasting urea, and injecting or banding urea-ammonium nitrate (UAN) solutions. Growing season applications can help reduce the potential for leaching and denitrification.

Importance of Nitrogen in Corn

Nitrogen is an essential nutrient for corn growth and maximization of yield potential. Plants with N deficiency can be easily identified by a characteristic yellowing on younger plants, and an inverted “V” shaped yellowish discoloration starting from the tip of the leaf up to the midrib in mature plants.

Nitrogen shortages can appear quickly in seedling corn, if N was not applied prior to planting or due to environmental conditions such as cold and wet or extremely dry can prevent N uptake. Corn plants require an adequate amount of N starting at the V5 (5 collared leaves) through V8 growth stage because the number of potential ears and ear girth is determined during this growth period. The quickest N uptake occurs from about the V8 growth stage through silking providing environmental conditions are favorable for plant growth (Figure 1).1


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Figure 1. Percent of total nitrogen uptake for corn by growth stage. Nitrogen data adapted from “How a corn plant develops,” Special Report 48, Iowa State University.


Impact of Nitrogen in Corn

The yield potential for corn plants not fertilized with N can be greatly reduced as plant-available N in the soil becomes depleted. Research conducted in Iowa demonstrated an average yield of 60 bu/acre for continuous corn and 115 bu/acre for corn following soybean, when additional N is not applied.2

Within the plant, N is critical for photosynthesis and production of amino acids, proteins, and chlorophyll. Without chlorophyll (which gives the plants their green color), photosynthetic processes and potential yield are compromised and reduced, respectively. Applying all N preplant has the risk of being lost to the environment through leaching or denitrification. Lost N is not only unavailable for crop growth but becomes an environmental concern when it enters waterways and the air. Sidedress and pre-tassel N applications can help reduce environmental concerns associated with N loss, supply N closer to maximum crop uptake, and allow for adjusting N rates depending on current growth, plant population, and environmental forecasts.

The majority of N should be applied just before rapid vegetative growth begins. For split sidedressing applications of N, Mississippi State University recommends applying no more than 1/3 of the total N at crop emergence and then applying the remaining N amount about 30 days later when corn is at or beyond the V6 growth stage.2 UAN liquids are applied to the leaves.


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Figure 2. Applying anhydrous ammonia as a sidedress.

Sources of Nitrogen

Urea-ammonium nitrate (UAN) and anhydrous ammonia are good N fertilizers to use when sidedressing corn.

.Urea-Ammonium Nitrate. Early-season sidedressing can be accomplished with an applicator that injects N (Figure 2, page 1) or can be surfaced applied in the form of urea or 28% or 32% UAN liquid solutions. Surface applied N should receive rainfall or an irrigation, or be tilled in soon after application to avoid volatilization which can be up to 30% within two weeks if temperatures are warm. Urea granules can burn the leaves when they fall into the plant whorls or when UAN liquids are applied to the leaves (Figure 3).

To help reduce the risk of burning leaves, UAN recommendations include:

  • Applying less than 90 lbs UAN/acre when corn is at growth stages V3 or V4.
  • Applying less than 60 lbs UAN/acre at V7 growth stage.
  • Drops should be used after V7 through pre-tassel nitrogen (PTN) to apply a concentrated UAN band application at an N rate that supports current yield potential.

Anhydrous Ammonia. In general, injection of anhydrous ammonia is potentially the best sidedressing choice. Best management practices (BMP) for applying anhydrous ammonia as a sidedress include:

  • Applications should be to the moist, not wet soil.
  • Applications should occur prior to root expansion into the row middles to avoid root damage.
  • Applications can begin immediately after planting.
  • Injection can be between every row or every other row.
  • To avoid snapping off plants, applications should occur before plants are too tall for the tool bar.
  • Applicator safety precautions should be followed to avoid injury to eyes and limbs.

Pre-tassel N (PTN) application

Another time period for a growing season N application is when the plants enter the V10 growth stage and just prior to tasseling (VT). An application during this period is referred to as PTN. Recommended PTN application rates range from 15-25% of the total N to be applied. The other 85-75% of total applied N should have already been split between a preplant injection or a sidedress operation. Equipment such as a 360 Y-Drop® applicator should be considered for a PTN application to place N near the plant base and to reduce the potential for plant breakage (Figure 4).

Should weather conditions prevent a sidedress application, a PTN application can help supply needed N. University trials have demonstrated that a late PTN application can improve yield potential.3


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Figure 3. Corn leaf damage from broadcast UAN at 100 lbs N/acre. Photo courtesy of John E. Sawyer, Agronomy Extension, Iowa State University.

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