Corn nitrogen recommendations

Kurt Steinke<ksteinke@msu.edu>, Michigan State University Extension, Department of Plant, Soil, and Microbial Sciences - April 13, 2017

Optimal nitrogen rates are not closely related to grain yield.

Variable—the one word often used to describe corn response to nitrogen (N). Soil variability within a field influences the N supply due to changes in soil texture, organic matter content and topography while environmentally both temperature and rainfall distribution are often key drivers of N variability across entire landscapes. Despite new application technologies and a plethora of products targeting improved plant growth and efficiency, three factors including rainfall frequency, crop heat units and soil texture still appear to offer the greatest influence on corn response to N.

Resources are available to reduce grower uncertainty when selecting corn N rates. Seven states through the Corn Belt, including Michigan, utilize the Maximum Return to Nitrogen (MRTN) corn N recommendation system. The MRTN model was adopted to further enhance farm profitability by maximizing the economic return of N fertilizer invested while simultaneously addressing some of the negative environmental consequences that occur when applying excessive N rates.

The model provides a range of N rate recommendations based on corn yield response to N over many years and across a range of Michigan soils. What the MRTN model also recognizes and accounts for is that the most economically optimum nitrogen rate will never be constant as corn and fertilizer prices fluctuate over time. The model provides a profitable range of N rates that allows for user input to adjust rate based on crop rotation, soil productivity potential and current price of N fertilizer and corn grain.

The MRTN recommendation table (below) is a summary of results from the Michigan database within the Corn Nitrogen Rate Calculator. Both tools may be accessed via the Michigan State University Soil Fertility Research website.

Using the MRTN recommendations

MSU Extension recommends following these important notes when using MRTN recommendations.

The MRTN model is a pre-season general N recommendation model that provides corn N response data that have proven profitable over many years and accounts for optimal and sub-optimal growing seasons. The model does not account for individual site variability or variable in-season weather events (i.e., individual large rainfall events or excessive rainfall after early N applications), which may affect corn N response and require adjustments to in-season N applications.
Corn yield for the N rates listed at the 0.05 price ratio will be near maximum levels, but N rates for greater price ratios may result in a greater economic return to the grower.
When the previous crop is soybean, the N credit is built into the recommendation system. Do not take any additional N credit as the rotational effect of soybean is already accounted for under the “previous crop” heading.
If the previous crop was a small grain that was interseeded with a leguminous cover crop species, growers should follow the recommendation category for previous crop soybean and small grain. If no leguminous cover crop was used with the small grain, growers should default to the recommendation category for previous crop corn.
Nitrogen credits for previously applied manure are not accounted for in the MRTN table. Manure N credits need to be subtracted from the recommendations listed in the table.
The profitable range listed beneath suggested N rates can be used to adjust N 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.

Suggested N rates for corn grain grown in Michigan, 2017
Soil productivity potential¹	Previous crop	N:Corn price ratio
		0.05	0.1	0.15	0.2
		Suggested N rate (lbs. N/acre)
High/very high	Corn	185	170	155	145
	Corn	170-200²	160-185	145-170	135-160
	Soybean³ and small grains⁴	160	145	130	120
	Soybean³ and small grains⁴	145-175	135-160	120-145	110-135
Medium/low	Corn	155	145	135	120
	Corn	140-170	135-160	125-150	110-135
	Soybean³ and small grains⁴	130	120	110	100
	Soybean³ and small grains⁴	115-145	110-135	100-125	90-115
Loamy sands and sands	Irrigated – all crops	215	195	180	170
(CEC less than 8.0)	Irrigated – all crops	200-230	180-210	165-195	155-185

Data from Kurt Steinke, MSU, 2017.

¹ Low: average yield is equal to or less than 135 bushels per acre.
Medium: average yield equals 136-165 bushels per acre.
High: average yield equals 166-195 bushels per acre.
Very High = more than 196 bushels per acre (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.

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