Nitrogen Management for Corn following Corn

Larry Bundy, Dept. of Soil Science

High corn prices and increasing demand for corn grain for use in ethanol production have stimulated producer interest in producing high yields in corn following corn cropping systems. Using appropriate nitrogen (N) rates and management is critical for successful corn on corn production.  Although high yields are the objective for most producers, it is important to recognize that desired yield or yield goal is not a good predictor of the optimum N rate in corn production.  Effective N management includes selecting the N rate that will maximize economic return and using appropriate application methods and management techniques to minimize losses of applied N.  Selecting the appropriate N rate is the most important N management decision because it has major effects on agronomic performance and risk of N loss to the environment.

The maximum return to N rate (MRTN) guidelines introduced in 2006, provide the starting point for the N rate decision.  As shown in Table 1, MRTN rates for corn following corn depend on soil yield potential and the N:corn price ratio.  Current prices place the N:corn price ratio near 0.10, and near-maximum agronomic yields are likely to occur at the 0.05 price ratio.  For high yield potential soils at the 0.10 price ratio, the MRTN rate is 135 lb N/acre, and rates that will provide an economic return within $1/acre of the MRTN range from 120 to 155 lb N/acre.  At any given N:corn price ratio, applying more N that the suggested rates is not likely to result in a profitable corn yield increase.

The range of rates shown for each price ratio in Table 1 indicates the range in which economic return to N will be within $1/acre of the MRTN rate.  Depending on the production situation, growers may wish to select the higher or lower end of the range provided [See Extension publication Nutrient application guidelines for field, vegetable and fruit crops (A2809), page 31 for further information].  For example, the high end of the range should be selected if corn residue cover will be greater than 50% at planting.  Adding N fertilizer to residue in no-till or other high residue systems for the purpose of promoting corn residue decomposition has been proven ineffective and often promotes N losses if a urea-containing fertilizer is used.

The MRTN rate guidelines must be adjusted for any non-fertilizer N sources such as manure applications, and second-year legume N credits that may be applicable.  For second-year corn following a good or fair alfalfa stand, a credit of 50 lb N/acre should be taken.  Appropriate legume and manure N credits are shown in A2809, page 60.

Table 1.  Suggested N application rates for corn following corn (grain) at different N:corn price ratios.

H  Rate is the N rate that provides the maximum return to N (MRTN).

I Range is the range of profitable N rates that provide an economic return to N within

   $1/acre of the MRTN.

' These rates are for total N applied including N in starter and N used in herbicide

   applications.

Once the MRTN rate has been selected and adjusted as described above, appropriate management strategies must be applied to control losses of available N (See A2809, page 38).  These strategies include placement, timing, and N source variables.  Where urea or urea-containing solutions (28 % UAN solution) are used as surface-applied N sources, growers should be aware of the risk of N loss through ammonia volatilization.  Maximum losses of N as ammonia seldom exceed 25% of the applied N, but these losses can be great enough to influence yield.  Inject or incorporate urea-containing fertilizers to minimize ammonia losses or consider using fertilizers that do not contain urea if N must be surface-applied.  Preplant or sidedress times of N application can be used on medium-textured soils with good drainage.  Sidedress or split sidedress N applications are preferred on poorly drained soils and are essential for coarse-textured sandy soils.

There is some uncertainty about the adequacy of N fertilizer supplies if demand is greatly increased by an expansion of corn acreage.  If supplies of N are limited, the following considerations can be used to prioritize N use.

• Apply some N to all potentially responsive acreage.
• Fully credit N from legumes and manure.
• Incorporate or inject manure to conserve N. 
• Reduce or eliminate N applications in cropping situations that need little or no fertilizer N such as corn following alfalfa and heavily manured fields.
• Use diagnostic tests, such as soil nitrate tests, to identify N fertilizer needs.
• Manage N to avoid losses.