Nitrogen (N) is typically the most yield-limiting nutrient for corn production and represents one of the largest economic inputs associated with maximizing returns. High N fertilizer prices together with recent N losses from excessive rainfall have placed N management at the forefront of corn production issues in many states. Growers there are questioning previous N management practices and are keen to adopt new strategies for improving N use efficiency in their operations.
Newly developed strategies for determining N rates and improving N use efficiency in corn production include the latest procedures for estimating economically optimum nitrogen rates (EONR), known as the maximum return to nitrogen (MRTN) method, along with soil- and crop-based approaches for determining EONR
Difficulties in Determining Optimum N Rates
There are several causes for the low N use efficiencies often experienced in corn production. These are frequently associated with N losses from the system, primarily resulting from excessive rainfall. However, another primary reason is the inability to accurately estimate EONR, the point where the last increment of N returns a grain yield increase large enough to pay for that N. This uncertainty occurs because corn exhibits a highly variable yield response to fertilizer N application due to highly variable weather conditions.
Inaccurate EONR estimates result in over-fertilization in some years and fields and under-fertilization in others. The former scenario reduces grower profits through wasted fertilizer inputs and can also lead to environmental contamination. The latter situation reduces grower profits through unattained yield goals. Hence, there is an undeniable need to improve N fertilizer management, but the ability to accurately estimate EONR on a year-to-year and field-to-field basis remains elusive.
Yield-Goal Approach to N-Rate Decisions
Previous methods for developing N-rate recommendations have often relied on the “yield goal” approach. This method begins by establishing a realistic yield goal based on field history and/or current capability. The amount of N required by the targeted yield is then determined from crop nutrient-removal tables (with a percentage added to account for losses). Finally, N “credits” from manure, preceding legume crops and soil organic-matter mineralization are subtracted from the requirement. The result is the amount of N fertilizer that must be added to supply crop needs.
Also referred to as the “mass balance” approach, this method is simple and holds considerable appeal, but it is not without its shortcomings. The major weakness inherent in this approach is that the relationship between yield and EONR determined by this method appears to be very poor for humid regions of the Corn Belt. This is a compelling limitation, considering that all Midwestern states east of the Great Plains are generally considered humid.
A summary of nearly 300 research studies revealed the limitations of the mass balance approach to N-rate determination. Researchers analyzed data from 298 previously reported experiments in five Corn Belt states in the U.S. where corn yield response to N was measured. Their investigations showed that the recommended N rates determined by yield goal exceeded EONR by an average of 80 lb. per acre. These results strongly support the notion that yield goal is a poor forecaster of corn N needs. Similarly, researchers in Iowa, Kentucky, Pennsylvania, Ontario and Wisconsin also found that yield goal was a poor predictor for optimum N rate.
An explanation for these observations is that years with very favorable growing conditions (adequate but not excessive precipitation) also have favorable conditions for N mineralization from soil organic matter. Conversely, years with poor growing conditions tend to have lower rates of mineralization. Because more N is supplied by the soil in productive years, the rate of N required per bushel of yield is less compared to the years with low mineralization. The net result is that corn requires about the same amount of fertilizer N regardless of the yield potential in a given year.
To learn more about measuring your N levels in-season, click here.