Developing an efficient corn nitrogen management program is difficult. Nitrogen is an elusive nutrient that can be lost from the crop/soil system through many processes such as fixation by weeds, bacteria, other non-crop organisms and soil, leaching through the crop root zone, gaseous atmospheric losses and erosion.

Plant-available nitrogen can come through many pathways such as soil organic matter mineralization, atmospheric deposition, atmospheric fixation (legumes), irrigation, manure, and other waste material application and commercial fertilization. Given the complex nature of nitrogen within the corn-growing season, we often experience poor nitrogen use efficiencies (amount of nitrogen added compared to actual amount used by the crop). There are some in-season tools we can use to estimate the successes or failures associated with current nitrogen management.

August is obviously too late to address this year's corn nitrogen needs. However, we can evaluate corn plants themselves to determine if the 2005 nitrogen management program was optimum for the environmental conditions that occurred this season.

A common way to evaluate late-season corn nitrogen status is to test the nitrate concentration of cornstalks. The time to analyze cornstalk nitrate content occurs during the 3 weeks following physiological maturity (black layer formation). At black layer, all kernels have reached maximum dry weight, and corn grain moisture is between 30 and 35 percent. Unused nitrogen at this time will not move into the ear and ends up being stored as nitrate nitrogen in the lower cornstalk.

The late-season cornstalk nitrate test is relatively more important for showing excessive nitrogen compared to deficient nitrogen. This is because there are distinct visual nitrogen deficiency symptoms for corn but no true visual characteristics associated with excessive corn crop nitrogen uptake.

Stressed or injured corn generally processes nitrogen very poorly, leading to high concentrations of cornstalk nitrates. For this reason, under stressed conditions, the nitrate test will not be accurate in determining optimum long-term nitrogen management for a given location. However, under stressed conditions the test will help identify if corn nitrate levels are safe to feed. Dr. Jim White, MFA's staff ruminant nutritionist, is concerned about feeding high nitrate (greater than 2,500 parts per million) cornstalks. If your samples are above this range, then consult an animal nutritionist before feeding. Fortunately, the nitrate level in cornstalks that will harm animals is greater than that needed for optimum corn grain yield production. Therefore, under non-stressed environmental conditions, proper nitrogen fertilization programs generally do not lead to animal health problems.

 Under "normal" corn growing environmental conditions, the cornstalk nitrate test will help producers determine whether the credits they gave for fertilizer, irrigation water, legume, manure and soil test nitrogen were adequate. The test can also play an important role with progressive producers that are variable rate applying nitrogen based upon management, past yield maps, soil types or other spatial components of the landscape.

Cornstalk nitrate sampling procedure

Cut an 8-inch segment from each cornstalk between 6 inches and 14 inches above the ground. Remove any leaf material from this segment.

Do not sample injured, diseased, barren or small-eared plants.

Do not allow a sample to represent too large of an area.

Keep stalk segments cool and clean while sampling. If samples are not mailed to a laboratory within 24 hours, refrigerate (do not freeze).

Ship each sample (composite of 15 stalks) in a paper bag. Sealed plastic bags will lead to mold or decay during shipping.

Laboratory results are usually reported in parts per million (ppm) nitrate-N. Different laboratories have different rating systems, but most place cornstalk nitrate levels into three or four general categories. The table below lists the cornstalk nitrate ratings from the University of Nebraska.