Corn nitrogen guidelines and focusing on the right rate

Kurt Steinke<ksteinke@msu.edu> and Jeff Rutan, Michigan State University, Department of Plant, Soil, and Microbial Sciences - April 04, 2019

Optimal nitrogen rates are not closely related to grain yield.

Weather (e.g., precipitation frequency and intensity), soil texture, temperature, soil physical properties, topography and soil depth are just a few of the variability factors that may add to the uncertainty when determining optimal corn nitrogen (N) rates. Nitrogen supply and recovery are critical components within overall nitrogen strategies that address the economic, social and environmental concerns regarding nutrient management. Despite claims of having corn nitrogen rates “locked in”, year-to-year relationships between optimal nitrogen rates and corn grain yield are often quite poor or non-existent.

Resources are available to reduce grower uncertainty when selecting corn nitrogen rates. Michigan in addition to six other states throughout the Corn Belt utilize the MRTN (i.e., Maximum Return to Nitrogen) corn nitrogen recommendation system. The MRTN is a pre-season static nitrogen guideline model based on corn response to nitrogen fertilizer applications across many years and locations while simultaneously accounting for fertilizer and grain investments. The model provides a nitrogen rate and profitable range adjustable by growers based upon crop rotation, soil productivity potential, current price of nitrogen fertilizer and corn grain, and field history.

In a new video hosted by Jeff Rutan, a post-doctoral research associate in soil fertility and nutrient management, Rutan focuses on the rate component within the four R’s of nitrogen management. Although split-nitrogen applications are a recommended best management practice in Michigan to improve nitrogen recovery, growers must determine total nitrogen needs to balance in-season nitrogen rates. In addition to MRTN, the video reviews other decision support tools including active crop sensors to detect nitrogen stress and guide site-specific nitrogen rates. Concluding the video are results of a two-year study at Michigan State University which utilized the MRTN approach and active crop sensor model to generate in-season corn SD nitrogen rates. The video, “Using Models to Predict Corn Nitrogen Rates,” can be accessed via the MSU Soil Fertility and Nutrient Management Program

Important notes to remember when using the MRTN guidelines

The MRTN model is a pre-season general nitrogen recommendation model that provides corn nitrogen response data that have proven profitable over many years and accounts for both optimal and sub-optimal growing seasons. The model does not account for individual site variability or variable in-season weather events (e.g., individual large rainfall events or excessive rainfall after early nitrogen applications), which may affect corn nitrogen response and require adjustments to in-season nitrogen applications.

Corn yield for the nitrogen rates listed at the 0.05 price ratio will be near maximum levels, but nitrogen rates for greater price ratios may result in a greater economic return to the grower.

When the previous crop is soybean, the nitrogen credit is built into the recommendation system. Do not take any additional nitrogen credit as the rotational effect of soybean is already accounted for under the “previous crop” heading. However, nitrogen credits for previously applied manure are not accounted for in the MRTN table and should be subtracted from the recommendations listed in the table.

If the previous crop was a small grain that was interseeded with a leguminous cover crop species, follow the recommendation category for previous crop soybean and small grain. If no leguminous cover crop was used with the small grain, default to the recommendation category for previous crop corn.

The profitable range listed beneath suggested nitrogen rates can be used to adjust nitrogen rates based on an individual grower’s familiarity with a specific field (i.e., tendency to yield greater or less than expected), the amount of risk a grower wishes to assume, or locally important air, soil and water concerns.

Table 1. Suggested nitrogen rates for corn grain grown in Michigan, 2019.
Soil Productivity Potential¹	Previous Crop	N:Corn Price Ratio
		0.05	0.10	0.15	0.20
		Suggested N Rate (lbs. N/acre)
High/Very High	Corn	195 180-210²	170 160-185	155 145-170	145 135-160
High/Very High	Soybean³ and small grains⁴	170 155-185	145 135-160	130 120-145	120 110-135
Medium/Low	Corn	165 150-180	145 135-160	135 125-150	120 110-135
Medium/Low	Soybean³ and small grains⁴	140 125-155	120 110-135	110 100-125	100 90-115
Loamy Sands and Sands (CEC < 8.0)	Irrigated – all crops	215 200-230	195 180-210	180 165-195	170 155-185

¹ Low: average yield = < 135 bu/A; Medium: average yield = 136 to 165 bu/A; High: average yield = 166 to 195 bu/A; Very High = more than 196 bu/A; (average yield is the five-year running average disregarding unusual highs and lows).

² Range approximates + $1 of the maximum return to N (MRTN) rate.

³ When the previous crop is soybean, the nitrogen credit is built into the recommendation. Do not take any additional nitrogen credit. Nitrogen credits for previously applied manure need to be subtracted from the N recommendations.

⁴ Refers to small grains interseeded with leguminous cover crop species. Small grains not interseeded with leguminous cover crop species should default to previous crop corn.

Dr. Steinke’s work is funded in part by MSU’s AgBioResearch.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).