Post Harvest Nitrogen in Stover - Where Does the N Go?

By: Bob Gunzenhauser

As harvest approaches, thoughts may turn not only to the grain that is being harvested from the field, but also to the remaining stover left behind.  Oftentimes the thoughts are about how to manage the residue in preparation for next year’s crop, perhaps through tillage or other means.

However, that corn stover can also be used as a scorecard for how well our nitrogen management played out in the previous crop season.

Let’s break down the numbers: For a 240 bushel per acre corn crop, assuming a harvest index of 0.50 (50% of above-ground biomass is grain, 50% is stover), about 11,400 pounds of stover (0% dry matter) is left in the field.  This will include stalks, leaves, cobs, husks, and tassels.

As we discussed in a previous article, during the final days of grain filling, the corn plant is remobilizing nitrogen from all parts of its organs into the grain – including the aforementioned.  Depending on how much nitrogen was available to the plant during the growing season, more residual N may be concentrated in the dried leaves than any of the other organs.  Studies have found leaves will have a range of 1.0 to 1.8% nitrogen concentration (dry matter basis) upon harvest and have about 20-30% of the total stover biomass.  Stalks will likely have lower nitrogen concentrations, usually at 0.3 to 0.7% nitrogen but represent 40-50% of the stover biomass.  Cobs and husks also have a similar nitrogen range as stalks and represent 15-20% and 8-11% of the biomass, respectively.

Percent of Total Stover Biomass, Nitrogen Concentration, and Estimated Nitrogen Pounds per Acre of Corn Stover by Organ

Organ	% of Biomass	Nitrogen %	N Pounds per Acre*
Leaves	20 to 30%	1.0 to 1.8%	23 to 62 lbs/ac
Stalks	40 to 50%	0.3 to 0.7%	14 to 40 lbs/ac
Cobs	15 to 20%	0.3 to 0.7%	5 to 16 lbs/ac
Husks	8 to 11%	0.3 to 0.7%	3 to 9 lbs/ac

*240 bu/ac grain corn yield, or 11,400 pounds per acre of stover on dry matter basis.

Corn genetics that have a higher “stay-green” rating will likely have higher concentrations of nitrogen in the leaves at harvest, while genetics with greater nitrogen remobilization ability will likely have lower leaf N concentrations post-harvest.

Residual nitrogen as nitrate is a measurement that can be made on the stalks before harvest as an indicator of nitrogen concentration in the plant and provide guidance for future nitrogen management.  The Corn Stalk Nitrate Test (CSNT) was developed by Iowa State University and can be used to help determine nitrogen usage and efficiency.

Taken as an 8-inch segment six inches above the ground, the analyzed stalk segment provides an indication of how much nitrate-nitrogen remains in the stalk and not used towards grain development.  Concentrations of less than 250 ppm of NO3-N indicate a nitrogen deficiency; 250 to 2,000 ppm is considered sufficient and amounts greater than 2,000 ppm are considered high or excessive.  The lower segment of the stalk is used because this is the last reservoir of nitrogen that the corn plant can pull from; if this is highly concentrated in nitrogen it indicates the corn plant was a luxury feeder of nitrogen, taking in more than it needed to supply to the grain.  See CROP 3154 Use of the End-of-Season Corn Stalk Nitrate Test in Iowa Corn Production for more details.

Overall, the corn stover nitrogen concentration, from all the various organs, will generally be 0.6 to 0.7% nitrogen.  Given 11,400 pounds per acre of dry matter biomass for a 240 bushel/acre crop, this would be around 68 to 80 pounds of nitrogen per acre returned to the field as stover.  This nitrogen is slowly released depending on the organ; leaves break down quickly due to the higher nitrogen concentration, while stalks and cobs may remain for years after harvest, depending on tillage and biological activity.  If this stover is partially harvested, reduced amounts of nitrogen will be returned.

Summary

Corn stover isn’t a waste; loads of nitrogen, potassium, and other nutrients are in the stover, not to mention carbon that can be incorporated back into the soil.  Knowing the amount of biomass and nitrogen concentration of various organs within the corn stover provides guidance as to what sources will become available faster than others for next year’s crop.  Knowing where nitrogen may concentrate, perhaps in higher than usual amounts than others, can be indicative of nitrogen management this year that can be used for future guidance.  In any case, your corn residue isn’t trash; it is a resource to manage properly.  Knowing more about it will help you make the right decisions.