Frost Injury & Nitrogen Management In Corn

By Rasel Parvej

LSU AgCenter

Most of the corn acreage in Louisiana was injured by freezing temperature from March 19-20. At that time, most corn is at VE (emergence) to V2 stage (two leaves with visible collar) and therefore the growing point was below the soil surface. Corn’s growing point remains below the soil surface until V6 growth stage for most hybrids. Since frost injury is usually limited to aboveground plant parts, unless the temperature falls below 28°F, the growing point was most likely protected from freeze injury. However, temperatures may have reached the critical temperature of 28°F in some areas, so each field should be scouted to ensure that growing points are still viable. At this point producers need to wait 5-7 days or until visible growth is observed to assess stand quality and uniformity. Many producers are inquiring about applying some nitrogen (N) fertilizer to jump start new growth. The following things should be considered prior to applying N in frost injured corn field.

If a corn field has already received 30-45 lb/acre N fertilizer at or near planting, extra N is not necessary at this growth stage (V2-V4), and they can wait to sidedress the bulk of their N around V5-6 stage. In silt loam to clayey soils, most of the applied N is still available in soil because temperatures have been cool, and we have not received excessive amounts of rain over the past few weeks. For sandy soils, extra N may be needed if several rain events have occurred that could have resulted in leaching N out of the current root zone.

If a corn field has not received any N or received a small amount of N, 5-6 lb/acre from 4-5 gallon of 10-34-0 fertilizer at or right after planting, producers need to apply 30-45 lb/acre N once they confirm their corn stand. Since urea (46-0-0) fertilizer price is going down, many producers are thinking about flying urea at this stage. Producers may broadcast small amount of urea to provide 30-45 lb/acre N, but they should not apply high rate of N by broadcasting urea without incorporation due to several potential loss mechanisms.

Injecting liquid N (urea ammonium nitrate, UAN 32-0-0, 30-0-0-2S, or 28-0-0-5S) near the root system is still preferred and most efficient method of providing N for profitable corn production. Corn root system is very small right now (VE to V3) and N is therefore needed close to the root system. Broadcasting urea (46-0-0) in a 36- or 38-inch row spacing system can not ensure enough N availability near the root system. Also, most of the urea granules when broadcasted fall in the furrow, which can be lost due to volatilization, leaching, or runoff before plant uptake.

Many producers have inquired about applying the total corn N needs by broadcasting urea. Reasons include: urea is less expensive, the time required to inject UAN on large acreage, concerns of compaction, or unsuitable soil conditions. The most efficient way would be to broadcast all the urea around V5-6 stage on dry soil followed by immediately incorporating it using a hipper. However, this may not be feasible for many producers due the time and above-mentioned soil conditions. Using any urease inhibitor will give the urea extra protection from volatilization loss as well as the producers extra time for incorporation.

The next best way for those who cannot incorporate urea by hipper would be to broadcast urea in 3-splits and each time shortly before a “known” rain event (at 0.25 to 0.5-inch) to help minimize loss mechanisms. Producers may also need to increase the total N rate up to 25% to compensate some losses depending on soil type and weather conditions. For example, when N is incorporated either by injecting or rehipping, LSU AgCenter recommends 200 lb/acre N for silt loam and 250 lb/acre N for clayey soils in 2-splits (30-45 lb at planting and rest at V5-6 stage) for 200-bushel/acre corn yield. The ideal 3-way split for urea should consist of 30-45 lb/acre N at VE-V2 stage, most at V7-8 stage, and another 45-60 lb/acre N at V12-13 stage before tasseling. To better assess pre-tassel N rate considering N losses during the growing season, producers need to monitor tissue N concentration at V10-12 stage.

If you have any questions on loss mechanisms and why I suggest not applying all the N using urea at one short without incorporation please contact me at 479-387-2988 or mrparvej@agcenter.lsu.edu.