Official MSU corn nitrogen recommendations

Continued spring weather delays may warrant a closer look at sidedress applications.

With poor spring planting conditions encountered thus far and more cool wet weather predicted, questions have been coming in regarding the need to adjust corn nitrogen management for the remainder of the year. Poor planting conditions and, in some cases, excessive precipitation events have some growers questioning whether pre-plant nitrogen applications will be as effective as in years past. Complicating matters is that there still appears to be some confusion over the new corn nitrogen recommendation system that has been implemented over the last several years.

Maximum return to nitrogen (MRTN) versus yield-based recommendations

The switch to MRTN was primarily undertaken to further enhance farm profitability by maximizing the economic return of nitrogen fertilizer invested while simultaneously addressing some of the negative environmental consequences that occur when applying excessive nitrogen rates. The yield-based recommendation system demonstrated a linear response with regard to nitrogen rate and corn yield. Although many growers understood that an infinite increase in yield was not feasible, other factors including high corn yields with low nitrogen inputs made this system quite uncertain.

The MRTN system accounts for additional factors when determining nitrogen rates including soil productivity potential, previous crop, average yield, the current price per pound of nitrogen fertilizer and the expected corn price. Together, these factors may begin to identify the most economically optimum nitrogen rate in which the last unit of nitrogen applied returns a large enough yield increase to pay for that unit of nitrogen. What the MRTN system also recognizes and accounts for is that the most economically optimum nitrogen rate will never be a constant measure as both corn and fertilizer prices will fluctuate over time. Several nitrogen:corn price ratios were built into the recommendation system.

While the MRTN approach is a step in the right direction for both environmental stewardship and grower profitability, the system is still in its infancy and will need further modifications including an increased number of data-producing locations and additional crop rotation data.

Table 1. Suggested N rates for corn grown in Michigan using the MRTN approach, 2011.

Soil Productivity Potential1 Previous Crop N:Corn Price Ratio
0.05 0.10 0.15 0.20
Suggested N Rate (lbs. N/acre)3
High/Very High Corn and all crops except soybeans and small grain 170155-1852 150135-165 135125-150 120110-135
Medium/low Corn and all crops except soybeans and small grain 140130-155 130120-145 120110-135 10595-120
High/Very High Soybean and small grain 145130-160 120110-135 10595-120 9585-110
Medium/low Soybean and small grain 115100-130 9585-110 8575-100 7565-90
Loamy Sands and Sand(CEC< 8.0) Irrigated – all crops 210195-225 190175-205 175160-190 165150

1 Low: average yield = < 120 bu/A; Medium: average yield = 121 to 150 bu/A; High: average yield = 151 to 180 bu/A; Very High = more than 181 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 or small grain, the nitrogen credit is built into the recommendations. Do not take any additional nitrogen credit. Nitrogen credits for previous legume crops or applied manure need to be subtracted from the n recommendations.

Poor spring weather may place greater importance on sidedress nitrogen

Sidedress nitrogen applications have been proven to be effective across soil types and offer the grower several additional benefits, including:

  • Avoiding early season N losses from excessive precipitation
  • Allowing the grower to adjust total N rates according to soil nitrate testing (PSNT)
  • Improved N use efficiency by applying closer to the time of maximum N uptake
  • More accurately adjust yield potentials based on growing conditions
  • The capability to account for any inabilities to apply adequate N fertilizer at planting or specific N products due to poor field conditions.

The time for rapid nitrogen uptake typically occurs during a period from 6 to 12 weeks after planting. To ensure plant uptake, growers need to apply sidedress N applications no later than six weeks after planting. The risks of delaying sidedress N applications beyond six weeks after planting are far greater than applying N at or before this point in time.

See related article on Corn nitrogen rate calculator may improve grower profitability.

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