The unusually mild weather in early spring 2012 and the early planting that resulted, together with last fall's dry conditions in many areas, are presenting unique management challenges to corn growers who are making nitrogen applications this spring. These conditions can mean nitrogen loss which can impact corn yields.
"For a successful corn crop, it's important to factor in the uncertainties of weather and the dynamic nature of the nitrogen cycle," says John Shanahan, Pioneer agronomy research manager who has conducted numerous studies on nitrogen management. "After all, a sound corn nitrogen plan sets the stage for a high-yielding crop."
Variable weather makes it hard for growers to estimate how much N to apply
Last fall's dry soil conditions led to anhydrous ammonia being applied on many of the acres in the Corn Belt. In the spring, excessive rain can threaten these soil nitrogen reserves and hinder making nitrogen fertilizer applications by ground equipment. Excessively dry conditions in the spring also can prevent applied nitrogen from moving from the point of application in the soil to the root zone of the corn plants. Temperature and moisture conditions also can impact the amount of nitrogen mineralized from the organic matter of soils.
These highly variable weather conditions make it difficult for growers to estimate optimum nitrogen rates accurately. So how do you figure out how much nitrogen to apply per acre?
Evaluate the crop's nitrogen needs as well as how much N you may have lost
There are several ways to estimate nitrogen needs as well as nitrogen loss, notes Shanahan. These methods include:
* N-rate calculator – Extension agronomists and crop advisers in several top corn-producing states have embraced the process of N-rate determination for corn. The Maximum Return to N (MRTN) approach provides generalized or regional N-rate recommendations using a database compiled from a network of multiyear and multi-location N-rate field trials.
* Soil tests – Take soil samples to evaluate nitrogen levels in fields. Testing soils at least 12-inches deep can help indicate the amount of nitrogen currently available to the corn plant.
* Scouting fields – Check low areas of fields to see if there is water standing. If temperatures are cooler for 10 days and the soil is 55 to 60 degrees, growers can expect up to 25% of nitrogen applied might be gone in these areas. If it warms up to 75 to 80 degrees, nitrogen loss could be up to 96%.
* Optical sensors – Sensors mounted on nitrogen application equipment emit modulated light of the appropriate wavelength onto plants to measure how much is reflected back to the sensor. This measure of "crop greenness" correlates with the plant chlorophyll content. Estimated chlorophyll content also estimates crop nitrogen status.
* Aerial view – If growers have access to an airplane, they can see areas of fields that are more yellow than others are, with observations being instantaneous and no need to wait for lab results.
* Estimation – In lieu of soil testing or crop sensing, nitrogen loss can be estimated by answering key questions, including when nitrogen was applied, what form was used, how much was applied, what the field conditions were following the application.
"To determine which approach is best for a particular farming operation, growers should carefully consider the pros and cons of each approach, likely using a combination of strategies to maximize potential profit while minimizing risk," Shanahan says.