Maximum Return to Nitrogen (MRTN) Method for Nitrogen Recommendations for Corn.

Historically, N rate recommendations for corn across the USA Corn Belt used yield goal times a per yield (bushel) factor to derive N application rates. Nitrogen response trials conducted over many years and locations indicated that agronomic and economic optimal N rates were not correlated to grain yield. This indicates that variable and interacting factors, other than yield level alone, are responsible for needed N fertilization rate. Also, due to a fairly constant relationship between fertilizer and corn grain prices for many years, the economic effect of prices was not considered in recommendations. The study of N rate data from many trials and multiple states indicated that needed N fertilization rate for corn following soybean (SC) and corn following corn (CC) had stayed constant over a long time period (studies evaluated from the 1970’s to 2000’s). As a result, yield based recommendations were deriving too high N rates because attainable yields had increased over time (due to many factors), but needed fertilization rates had not concurrently increased. In an attempt to deal with these issues, and to have a consistent recommendation system across the Corn Belt (system but not rate), various rate recommendation systems were evaluated, with the Maximum Return To Nitrogen (MRTN) chosen as a most desirable approach – initiated in 2005 and now used in eight states across the Corn Belt. With the MRTN approach, N rate trials from many locations are used directly to determine response to N application and maximal economic rate(s). Each state developed a N rate trial database for SC and CC, with on-going trial updates. Some states subdivided databases by geographic areas or soils. The MRTN approach incorporates N fertilizer and grain price into the rate determination, and thus is adaptable to changing economic conditions. While the MRTN approach does not adjust for yearly factors affecting N response, the large N rate trial databases do provide for a predictive method to determine N rates into an unknown future, and represents needed N fertilization on the long-term basis. Until more refined systems are developed that allow for improved adjustment in rate and economic return between years, the MRTN is an acceptable system for minimizing economic N rate based production errors.